Used in an interesting way in Temple Of Elemental Evil. Towards the end of the game, your party can encounter a human adventurer deep inside one of the Elemental Planes. She explains that she came down there with her party, but they were killed and she barely escaped. She asks to go with you, and if allowed you'll find she's a decent-ish sorceress. But if in dialogue your characters detect something amiss, they can confront her about it. At this point she reveals that she's actually a half-succubus, and wants to come along with the party because she's bored. If you let her, she'll drop the pretense and switch back into her true form, making her a much more powerful companion.
Naturally parodied in Barkley, Shut Up and Jam: Gaiden. The first form is already one-winged. So what happens when it's time for its true form? A dinosaur with Barkley's face on it. And not just any dinosaur, but the body of Diablo from Primal Rage!
Dracula fills this role to the letter in almost every Castlevania game. In this series, he's more of an ultimate evil rather than just some vampire. See the quote at the top of the page.
Castlevania 64 has him turn into some kind of 50-foot dragon/centipede hybrid. The game as a whole wasn't the greatest, but you will say Oh, Crap the first time you see Drac's final form. And that's before he starts throwing miniature H-bombs at you...
In Castlevania: Portrait of Ruin, Dracula fights you with Death at first, then after beating on one of them for a while, he uses Soul Steal on Death himself, absorbing him and turning into a giant demon.
In Castlevania: Order of Ecclesia, this is notably avoided for the first time in decades, as the entirety of the final battle against Dracula is fought against the dashingly-handsome vampire we've known for so long. This may have been from the fact that the player is holding half of Dracula's power in the form of Dominus, which must be used to end the battle, but not before beating on Dracula for a good long time.Super Castlevania IV did something like this as well, but as his health neared its end, Dracula's abode starts crackling with electricity again and pulses a red glow, and Dracula's head gets replaced with a demonic SKULL. This is when he starts breaking out the scary flashing lightning.
And for the times when Dracula wasn't the final boss? Graham pulled a mutilated abomination throne on us akin to Symphony's Dracula; Chaos showed that it was a black orb with pulsing black prominence flares inside of a really chaotic room; Menace unrolled into a golem made of souls.
Cornell transforms into a huge muscular man-wolf with a human(ish) face.
Carmilla transforms into a hideous bat monster.
Zobek the Lord of the Necromancers transforms into a wizened lich. In the sequel he changes his look into a more intimidating Grim Reaper-like figure with a skull for a head.
Gabriel Belmont as Dracula calls himself Dracul "The Dragon" for a reason. With the aid of his dragon amulet, he can tap into his full powers to transform into a mighty dragon.
Parodied in the patchwork freeware game I Wanna Be the Guy, where after delivering the line "look upon my true form and despair!", Dracula's true form is revealed to actually be a Waddle Doo that goes down in one hit. Played straight in the final battle against The Guy. After killing his (small, humanoid) form, you think you've won, only for him to crash through the wall in the form of a giant, laser-spewing, spike-laden, ill-tempered head.
In Dragon Age: Origins, the boss mage Uldred at the top of the circle tower reveals his true form as a Pride Abomination.
Should you choose to fight her, Flemeth turns into a High Dragon for her boss battle. The second game implies that this might be her true form - at least, no-one has a clear idea what in Thedas she actually is.
The second game's Legacy DLC features an optional boss called Malvernis. He starts out as a spectral mage-like opponent, then turns into a ghostly dragon. Oddly, he turns into a flesh-and-blood dragon after you kill him.
Virtually every human or humanoid boss in Breath of Fire III did this. Mikba, Balio and Sunder, the Professor, Teepo, Rei - even the main character, Ryu. Just to hit the nail in further, when Balio and Sunder combined to form Stallion and when Mikba assumed his true form, they each said, "No one has seen us/me in this form and lived!" But considering this game's great love of random bosses, it's not surprising that every boss trope in the book was visited a few times. The rest of the the BoF games are just as full of transforming bosses.
Taken to an extreme in Wild ARMs 3, in which the final boss has a whopping ten forms. The first Wild ARMs game has a few of these. Several bosses will take another form after their apparent defeat, only to return bigger and badder than ever (so they claim) such as Mother, Ziekfried, and Boomerang (after his final death you can fight his resurrected form in the arena). Alhazad doesn't transform, but he has always worn a white cloak covering his body, and he finally takes it off to reveal his true form. But the best example in this game that fits the trope perfectly is Zed - a bumbling wannabe boss who, in a side quest late in the game, finally loses his cool and turns into his true form, a huge grotesque monster and becomes one of the toughest bosses in the game.
In Arc The Lad 2, every single human enemy will transform into some kind of monster or another before fighting the heroes.
In Super Mario RPG, Smithy transforms into another being after being stomped by Mario and his crew in the first battle because he is so ticked off that he lost and wants to crush Mario and his friends.
The main antagonist, Fawful, transforms into Dark Fawful, and during Bowser's battle with Dark Bowser, the Mario Bros. fight Dark Fawful in a giant form with many limbs that must be attacked (similar to the previous two final bosses before him). Although in actuality he is really quite small, since Bowser can inhale him.
In Ninja Gaiden for the Xbox and its Updated Rereleases, humanoid Fiend Alma turns into a scorpion-like thing once her power gets Awakened. After Doku sheds his corporeal shell, he loses his legs to float and gains a nodachi worthy of the One Winged Angel himself. The Vigoorian Emperor goes from a vaguely angelic statue to a bony creature made of skulls.
Ganon/dorf. In The Legend of Zelda: Ocarina of Time, you fight his phantom incarnation on horseback, then him personally first as evil sorceror-king Ganondorf, after which he calls upon the Triforce of Power to transform into the shadowy boar-monster Ganon.
In The Legend of Zelda: Majora's Mask, the final boss of the same name progresses from a mask with hair/tentacles (Majora's Mask) to a mask with arms and legs (Majora's Incarnation) to a giant, psychedelic, whips-for-arms demon (Majora's Wrath). It was also notable for allowing Link a supreme transformation into the Fierce Diety, only usable against bosses but still near-impossible to lose with.
Subverted in Oracle Of Ages by game Big Bad Veran: Her true form is human, but she immediately transforms into something that looks like a "mutant fairy", and her final "combat form" (which were actually three in one) is so ridiculous that she feels humiliated by having to resort to it.
General Onox of Oracle Of Seasons plays it straight by revealing his true form to be a giant skeletal dragon.
In The Legend of Zelda: Twilight Princess, Ganondorf transforms early in the fight. The twist being that, this time, Link does it too. Also inverted in that you fight Ganon's human form after you fight his monster form in this game.
This also applies to Vaati in The Legend of Zelda: The Minish Cap. At the beginning of the battle, he morphs from his human form into a taller, more powerful-looking version of himself, and after that's beaten, he changes completely into a giant, spherical beast with one huge eye and giant claws.
Bellum from The Legend of Zelda: Phantom Hourglass, however, is harder to classify. He starts out as a big, squid-like monster covered in eyeballs, then progresses to a possessed battleship. Once that's out of the way, though, he actually assumes a much smaller, humanoid form, via possessing Link's buddy Linebeck.
Malladus, the final boss of The Legend of Zelda: Spirit Tracks, plays this straighter than Bellum did, but is still a bit odd, in that he attains his last, monstrous form by taking over Chancellor Cole's body, mutating it in the process. In a strange coincidence, the result ends up looking somewhat like Ganon's Dark Beast form from Twilight Princess, only more goat-like than pig-like.
Then The Legend of Zelda: Skyward Sword goes and inverts it, having the final boss Demon King Demise fought several times in a monstrous, demonic form before assuming his true, humanoid form for the final battle. Meanwhile, Ghirahim turns into a Chrome Champion for his final battle. This is alluded to in the game when Fi mentions that his muscle mass went up by 90 percent in that form.
In Pokémon Black 2 and White 2, after Ghetsis fails to freeze the player alive, Kyurem absorbs N's Reshiram/Zekrom and becomes a flaming/electrified (depending on the version) man-eating ice dragon.
Oracle Of Tao frequently uses this trope. Not only does one of the middle game bosses do this, but the final boss of the game does it twice once during the normal conditions (two forms, unless you also count the one he abandons before the battle even starts), and again if you qualify for the Playable Epilogue (3 more). To say nothing of enemies that cheat the HP limit by having multiple attack patterns with the same form (one of those you have to kill nine times).
The "Devil Trigger" power of various Devil May Cry playable characters involves them turning into an uglier demonic form. This overlaps with Super Mode and Limit Break. More straightforwardly, bonus points to Credo for turning into a one-winged "angel".) Arkham of the third game goes from an already unpleasant humanoid demon form to an even worse blob form. His in game profile states that his blob form is a reflection of his inner evil.
In Spiral Knights, Lord Vanaduke does a variation of this. Although he starts out already big and monstrous, he takes the trope Up to Eleven in his final form. Upon significant damage, he will lose his mask and reveal his shadowed face, commencing the transformation. Fire and lava burst from cracks in his armor, he begins to glow as if possessed, and orbs of cursed flame spin around him, forming a halo as if he has become a god. In addition, he is surrounded by corrupted Slag Guards. The Jelly King/Royal Jelly does this as well, except he loses his crown to become more powerful, rather than gaining it.
The original Sonic Adventure has Chaos Zero, who actually has seven different transformations, one for each Chaos Emerald he absorbsnote He never transforms into Chaos 3 or Chaos 5, as he was given two Emeralds at once as Chaos 2 and 4, respectively. After absorbing all the Emeralds, he transforms from a water elemental-like mutated chao into a watery Leviathan powerful enough to destroy entire airships in one blast.
In Sonic Adventure 2, there’s the Biolizard, the prototype of the Ultimate Lifeform. After it was initially defeat by Shadow, its successor, it uses Chaos Control to warp right out of the room and binds itself to the Space Colony ARK note a giant space station housing, among other things, a weapon capable of destroying a planet and begins a crash course for Earth.
Sonic Heroes has this happen to Metal Sonic who, after spending most of his time in the shadows, transforms into a titanic metal dragon called the Metal Madness, armed with the power of the aforementioned Chaos, as well as that of Team Sonic and Shadow. Three-quarters into the battle, he sprouts huge metal wings, further transforming into the Metal Overlord.
Metal Sonic goes through a similar transformation at the end of Knuckles Chaotix.
Following a botched attempt to mind control the main protagonist, Black Doom, the main Big Bad of Shadow the Hedgehog, transforms into Devil Doom, which was a giant 2-headed dragon-like alien that melded itself into the Black Comet.
The handheld title Sonic Advance 3 has Gemerl. Following the initial final battle with him and Eggman, he swipes the Chaos Emeralds from Sonic, akin to Sonic The Hedgehog 3's intro cutscene, and uses them to transform into a berserk orb-like machine with laser-firing claws.
In Sonic and the Black Knight, there's Merlina, the Dark Queen. Throughout the course of the game, she goes from a harmless cute wizard to a scary evil sorceress to a giant armored knight with four arms, two of which are armed with building-sized swords.
Devan Shell transforms from a wimpy and nerdy turtle to a big winged turtle-demon at the end of Jazz Jackrabbit 2, appropos of nothing.
By beating the Adventure mode of Super Smash Bros. Melee on a hard enough setting, the last boss Bowser transforms into the giant beast that is Giga Bowser. note Specifically, this requires completing Adventure Mode in normal or higher difficulty within 18 minutes without using any continues.
Brawl actually has this as Bowser's Final Smash, Gigabowser; Ganondorf gets a similar transformation, based off his Twilight Princess beast form. Mr. Game & Watch's Final Smash has him transform... into an octopus.
In the Subspace Emissary in Brawl, Tabuu, the Man Behind the Man (behind the other men), transforms into a winged version of himself that can transform the heroes back into trophies. But then Sonic comes and damages both of his wings, thus weaking his power to just an instant KO.
Then there's Great Commander's second form from Star Fox/Starwing.
Phantron, which morphs into a jumping frog-like mecha for its second form, with a Scare Chord during the transformation.
In the first game's Hard route, Andross morphs into a horned demon about halfway through the battle.
Tales of Phantasia has Dhaos, who is first fought in his human form, then reveals his "True Power", a giant monster vaguely reminiscent of a giant armored purple-and-red Praying Mantis, then crosses the Bishonen Line by invoking his gods and turning into an angelic-like being -much like Yggdrasil from the later Tales of Symphonia. In the original SNES-version, he only has the first two forms, however.
Kronos/Miktran from Tales of Destiny turns into an Eldritch Abomination after being defeated once in the original game, but takes on a form resembling a purple demon in the remake.
The final boss of the main story of Tales of Legendia starts off looking human, but transforms after he absorbs the power of the Nerifes and becomes its physical avatar, which looks like a blueish humanoid robot.
Mathias from Tales of Innocence ditches her human form immediately, changing into a freakish centaur-like form that has her torso situated on the body of Asura in place of his head. In the remake, she additionally takes on a human form after that, then becomes a near perfect copy of Asura for her final form.
In Tales of Vesperia, Duke starts off looking perfectly human, then changes into an ethereal form with a ton of Attack Drones floating around him. If you collected all of the Fell Arms, he'll change into a third form, which is similar to the second, but with dark skin and no shirt.
Bizley in Tales of Xillia 2 chooses to activate his Corpse Shell mode for the final battle, which manifests as a suit of black and red armor with black wings.
All of the characters in the Bloody Roar series are capable of switching between normal human form and superpowered creature forms AND a glowing 'hyper' version of the beast form, but in "Bloody Roar: Primal Fury", the hyper-beast form of the true final boss, Uranus, is identical to her human form. Her beast form is a chimera, which is interpreted as blue and scaly with red lines, covered in spikes, and kind of bull-like, so yeah, it's a monster.
One character in Primal Fury's normal beast form...is a little, unimposing penguin. His hyper form is a seven foot tall phoenix-man perfectly capable of annihilating your world ten times over.
Subverted in the final battle of Mega Man 2; Dr Wily appears to morph into a green alien and the background changes to a starfield. After you defeat him, however, it turns out it was just a virtual reality machine.
Zero is pretty fond of this as well. Not only the various final bosses transform at least once, also the four elemental masters transform. Interestingly, in the third part it was played straight and inverted, the former between Omega's large first form to enormous second, and the latter to Zero form, with its own legitimate battle rather than Zero-Effort Boss scene (no pun intended).
While certainly not monstrous or one-winged, in X8 the True Final BossLumine not only triggers an angelic battlefield platform, but also grows six wing-like mechanical limbs from his back, now has bright red, slitted eyes, thin, long, and pointed fingers, and has a red crystal produting almost a foot out of his chest.
In Mass Effect 1, when you confront Saren for the last time in the ruined Citadel control room, he is initially in his (admittedly rather warped) normal form. However, after he is defeated or convinced to commit suicide (it depends on your dialog choices beforehand), the Reaper Sovereign uses its power to transform him into a large, spindly biomechanical monstrosity with exposed ribs and movements very similar to the earlier geth hopper enemy types, albeit far more dangerous. Whereas Saren's first form is a humanoid character not unlike your party members, his second is a highly mobile quadruped that vaguely resembles a metallic, skeletal lion. Only with the top half of his face still attached.
The initial design for the game, however, had this in mind. The art book for the game has concept art for the Illusive Man's massive, grotesque final boss form. Ultimately, it was decided this trope had no place in overall design because it goes against the Illusive Man's character as The ChessmasterMagnificent Bastard, thus leaving the game without a final boss.
Arcanum's final boss, if engaged in combat, transforms into a massive bone snake-dragon..thing. Although in this case, he started out as a cloaked figure (so the transformation didn't really make him more 'morally OK' to kill) and the dragon isn't really very good at fighting, so I guess he just thought it looked cool. He was supposed to be a lot tougher, but Troika accidentally flagged him as humanoid. Humanoids are hard-coded to deal no more than 10 points of damage in unarmed combat. Unofficial patch fixes it.
Kangaxx the demilich starts out as a moderately tough spellcaster, then transforms into a levitating skull that is immune to magic, immune to weapons of less than + 4 enchantment, regenerates, has an AoE instant death attack and casts some of the most powerful spells in the game, including 'Imprisonment', which not only causes instant almost irrecoverable death for the duration of the fight but can also break the plot.
The player gains the ability to turn into the Slayer, an avatar of Bhaal the god of murder and a Superpowered Evil Side. In the final battle, the villain, having co-opted the power of Bhaal, does the same.
Half-dragon, partly-divine Abazigal and his son Draconis both have both a humanoid and dragon form and turn into the latter after fighting in the former.
The final boss, again, is stealing the power of Bhaal for themselves and trying to become a god; they get as close as becoming an immensely powerful, giant version of themselves for the final battle(s).
Parodied with particular glee in Kingdom of Loathing, where the Naughty Sorceress reveals her supposedly true form. After you beat her in her hideous true form, she then assumes her actual true form: a sausage. Then again, she is an evil sausage brimming with dark magic. Your character then loudly proclaims, "How many times do I have to kill you? This battle has taken over a half an hour and there's no save point!" (Said sausage is a Clipped Wing Angel, as you jst have to have the Wand of Nagamar in your inventory to beat it.
Later in the history of the game, Ed the Undying was introduced, who had seven forms, each weaker than the last. Even people who hate the quest in which he's involved love his dialog. In Ed's case, though, it's not so much transforming as it is coming back with progressively more body parts hacked off.
Parodied with the Fallen Archfiends, a minor enemy in the Gate to Hey Deze. One of their "failed" attacks... well, a summary just wouldn't do it justice:
He glares at you and his arches glow a bright gold. "Now you will see my true... my true... ugh..." the fiend clutches his right arm, shouts "I'm comin', Elizabeth!" and falls down.
Played straight with the final, demon form of your Nemesis. As opposed to other examples of this trope, however, if you beat the first phase but lose to the demon phase, you can buff and heal yourself, change your equipment, even adventure elsewhere, and when you attempt the fight a second time you don't have to fight the first form again.
The Disc One Final Boss in Adventures of Rad Gravity, Agathos, is a human mutated into a giant living brain.
In Ys V, The Brute, Dorman and the Big BadSealed Evil in a Can, Jabir mutate into grotesque monsters for their boss battles; Dorman resembles Gadis's one-winged angel form from Dawn of Ys, while Jabir looks similar to Galvaran (he may be one of the Ash Emelas monsters).
In Ys VI, Ernst uses the power of the ark to become a "two-winged dark angel". After his defeat and subsequent return to normal form, he tries to draw more power from the Ark, but suffers a fatal Super Power Meltdown and causes Napishtim to go haywire.
Shining The Holy Ark: Panzer absorbs some of the evil from the Holy Ark to turn into a monstrous final boss - an interesting variation in that Panzer is actually The Dragon.
The Final Boss's first form in Monster Lair is a clone of the original Wonderboy riding a dragon, but for his second form he grotesquely morphs into a green "space ghoul" type monster. Horrifying, considering this is a rather "kiddish" game, although Nintendo Hard. However, this form is relatively easy compared to his first form and a few other bosses, thus it may be considered a Clipped Wing Angel.
The end boss of The Legend of Dragoon had four different forms, each one symbolizing one of the seven days of the creation of the world. None of them actually looked like anything in particular.
Giygas from Earthbound. At first, he looks like a giant eyeball with Ness's face on it, but this is just the result of using a machine to stabilize his form. After you damage Pokey enough, he turns off the machine and Giygas assumes his true form, which has to be seen to be believed. The initial Giygas fits the bill as more alien like, seen in preceding game Mother as a more humanoid alien with vaguely lengthy appendages, within a chamber attached to a huge machine. The previous information continues to apply.
In Mother 3, Miracle Fassad fights like an upgraded version of his previous fight. After dealing enough damage, however, he reverts back to his New Fassad form and reveals that he is a PSI user, using high level PSI, including Starstorm.
Skies of Arcadia First you fight Ramirez, then he sacrifices his life to control Zelos, the Silver Gigas, and then he crosses the Bishonen Line. Interesting in that the boss's original form is perfectly visible inside his translucent One-Winged Angel form.
All three of the major bosses in God Hand fall into this, since they generally appear humanoid but are actually extremely powerful demons. Fat, cigar-smoking, and inexplicably Mexican Elvis turns into a 20 ft tall gray giant with huge mouths for hands, another huge mouth and eyes on his stomach, and a very small featureless head., Succubus Shannon is actually Her own upper body with the lower body of a demonic cat thing and a giant eye on her face., and Bezel, who in human form looks like a guy in a suit with gray hair and skin and long pointed ears, but in demon form is First a worm with a fly-like upper body and blade-limbs, then a huge fly with blade limbs. Both forms have Bezel's face on their back.
The Tekken series has such boss characters as Ogre's transformed state True Ogre (which resembles Beast from Disney's Beauty And The Beast but with a snake for a left arm). In 'The Devil Within' subgame from Tekken 5 Ogre goes through even more transforming states. While you don't fight them always (never Jinpachi, sometimes Jin) in their normal forms prior to their transformed forms, Devil Jin and Jinpachi from Tekken 5 are transformed characters. They are for most characters the second last and final bosses. In both cases, they are infected with the Devil Gene, so if the Devil Gene is seen as the enemy, it is incarnating within two separate people. If Devil Jin defeats Jinpachi, he absorbs his power and transforms further, though he is not someone who can be played or fought.
The Mortal Kombat series has this in almost all of their later games including Zombie Liu King from Mortal Kombat Deception.
Mortal Kombat Mythologies: Sub Zero had Shinnok transform into a polygon demon once Sub-Zero takes the amulet, but fighting him in this form is optional, as a portal leading to the end of the game opens up anyway.
The boss of Monster Madness: Battle For Surburbia is Mr Huggles, a parody of Barney the dinosaur. At first, he attacks by singing and hugging. After you fight him his suit comes over revealing his hideous true form, a slimy Jabba The Hutt-like creature.
Primal has a main character who can turn into different monster forms.
The Shadow Hearts series does this a lot. In the first game, every human boss enemy transformed into a monster of some sort to fight you. Largely due to powers of Malice or a pact. Or both.
In the Gungrave series, many bosses become like this due to the series' Psycho Serum / phlebotinum / green rock, the Seed drug. All of the "Big Four" bosses Grave fights usually morph to a mutated "overkill form" in the second round or as soon as he encounters them. Oddly enough, the final boss of the second game doesn't transform, he just gets some new attacks during the second phase of the fight.
In Diablo, the Dark Wanderer character (Diablo in the Diablo 1 Warrior's body) slowly transforms into Diablo starting from the beginning, up until just before Act 3 is completed. Similarly, Baal, having taken over Tal Rasha's body in a similar way, slowly transforms him beginning with his release prior to the completion of Act 2, until the final form seen in the opening movie (and final battle) of the Lord of Destruction expansion pack. King Leoric, the Skeleton King, also underwent a similar transformation, though he was able to resist full possession by Diablo. The Warrior's use of the soulstone may have made him more vulnerable to this though. Prince Albrecht succumbed immediately though, similar to how Griswold instantly became a zombie.
King Leoric was able to resist because at the time Diablo had just reawakened in the Soulstone. Prince Albrecht could not resist because as an infant he had little if any willpower to resist. The Warrior fell relatively easy because most the deeds Diablo caused Tristan were perpetrated to strengthen Diablo as well as perpetuate an Evil Plan to attract a hero powerful enough to kill him in Prince Albrecht's altered form and who would think that they were able to imprison Diablo in there mind, body and soul. Diablo by this time became powerful enough to gradually takeover the Warrior's body gaining a much more powerful host.
Happens in Diablo III during the final fight in Act II, when Belial transforms into a larger version of himself when his health reached 10%.
Joka from Klonoa: Door to Phantomile has the ability to change between his normal floating-jester-balloon form and a conspicuously monstrous form which resembles some sort of sea monster. This form is invincible, but can only be used during an eclipse; he can be forced to revert by stopping the magically-caused eclipses taking place during his boss fight.
In Klonoa Heroes: Legendary Star Medal, he has a different 2nd form: "Flower Joka", which is his normal self, except flower-shaped instead of spherical. Again, he shifts between this and his normal form during the fight, this time at will; luckily, his Flower Joka form can be damaged as normal.
Klonoa 2: Lunatea's Veil features a variation: Leorina, thought by the heroes to be the Big Bad, is forced to go One-Winged Angel by the real Big Bad, the King of Sorrow. She's not the Final Boss, either. Surprisingly, the King of Sorrow does not go One-Winged Angel during his boss fight, instead preferring to attack you from within a weird orb thing.
In the second game, WilliamBirkin initially appears as a somewhat mutated man and keeps reappearing in progressively less human forms until by the end, he has degenerated into a mass of teeth, flesh, and tentacles. Mr. X initially appears as a large man, but grows a pair of massive claws by the end of the game.
In RE3, Nemesis first appears as a bazooka-wielding Mr. X-like humanoid, then sheds his Bad Ass Long Coat revealing his Combat Tentacles, then finally resembles a giant squid with legs. Later, in Code Veronica, Alexia appears as an ordinary human at first, but mutates into a hideous queen ant/human hybrid and then a dragonfly-winged thing by the end.
In Resident Evil 0 Dr. Marcus becomes a huge mass of leeches just before fighting the characters.
In Resident Evil 4, Mendez, Salazar, and Saddler all have hideous final forms. To make things rather more disturbing, they all seem to be in complete control of their mutations, unlike bosses in the previous games. Downplayed with Krauser, whose final form only changes his left arm, mutating into a bloodstained claw.
Resident Evil 5 continues the tradition in grand style. An interesting example is where this is forced by the hero with Albert Wesker. Realizing that the Bishonen Line has made him Nigh Invulnerable, Chris injects him with The Virus, and he ends up having to mutate into a perfectly killable monster to fight it off (yes, even his biggest mistakewasn't his own fault). There's also Ricardo Irving, who injects a mutagen given to him by Excella and turns into a hideous sea-dwelling, multi-tentacled monster, complete with NarmtacularLampshade Hanging:
Special mention has to go to Morpheus Duvall of Resident Evil Dead Aim. A Sephiroth pretty boy who is obsessed with beauty turns himself into a transgender electrical Tyrant, and then mutates into an absolutely enormous green blob with large claws, rib cage halves sticking out of its back and a malformed head popping in and out of its torso as it drags itself along at a lumbering pace.
Resident Evil 6's antagonists both become this. Derek Simmons becomes a barely humanoid jigsaw of flesh capable of morphing into one of several forms but in the final confrontation, takes on the form of an insectoid creature. Meanwhile, the other Ada/Carla Radames turns into a massive white blob of goo that overtakes the ship she oozed over, both the exterior and interior. The fight is spent in the interior having to avoid the goo-slathered walls which spawn either limbs trying to grab and absorb you, duplicates of her humanoid slime form and explosive forms. The only way to combat her was the use of nitrogen.
Parodied in BloodRayne. The final boss of Act 2 is General Mauler, a 10-foot tall Nazi cyborg with incredible durability. When you first empty his life bar, he collapses to the ground, then gets back up again, raises his arms high, triumphantly declares "You can't beat me that easily!'', then... promptly falls over dead.
Baten Kaitos has several examples. In the first game there's Geldoblame, Fadroh, and the final boss Malpercio (who started out as a giant monster to begin with). The second game has all of Malpercio's Afterlings. The oddest example would be one from the first game, in that after Kalas's Face-Heel Turn, he grows a second wing, as he only had one wing to begin with and had to make due with a mechanical prosthetic for the other. So... he goes from being a One-Winged Hero to a Two-Winged Villain.
Fadroh transforms into a huge, vaguely jester-looking giant monster with a crotch-mounted eye. His main special attack consists of leaning back and firing Eye Beams from his pelvis.
Played with in Metroid Prime; the titular monster does this prior to the events of the game (by creating an armored exoskeleton armed with missiles and various energy weapons), and inverts the trope when you fight it (reverting to its core form once the exoskeletal form is defeated).
Emperor Ing goes through two very bizarre alternate forms during the battle against it, with the second form being a cocoon for his final form. Also, the larval Chykka metamorphoses into its dragonfly-like adult form midway into the fight. Dark Samus has also undergone a serious mutation as a result of chowing down too much Phazon and is neigh invulnerable
Any time an enemy gets controlled by an Ing mid-fight (the Alpha Sand-Digger and Alpha Splinter are prime examples)
Metroid Prime 3: Corruption has Dark Samus merge herself with Aurora Unit 313 to provide the final boss battle.
In Kirby Super Star there's Marx, and Kirby 64 has both Possessed King Dedede and the final form of Dark Matter, sorta. And there's even a regular enemy, Scarfy, that pulls this rather terrifyingly.
Kirby Super Star Ultra manages to one-up on Marx in "The TRUE Arena" — if you get to the final battle, you will see a movie where Marx brings back his soul by absorbing Nova's power. You will then fight Marx Soul, which is a more powerful and freakier incarnation of Marx that has an absolutely terrifying death scream.
The final boss of Kirbys Return To Dreamland also pulls this twice, with Magolor switching into a much more grotesque form after the first form is beaten.
The final boss of Kirby Triple Deluxe does this, as well, having Queen Sectonia merge with the World Tree to become a gigantic insect/flower hybrid.
In Silent Hill 2, Maria transforms into a gray-skinned tentacled levitating upside-down-in-a-cage abomination for the final battle.
In the original version of Lunar: The Silver Star, the main villain would announce that it wasn't over after being defeated. He then turned into a rather stereotypical anime demon for no explicable reason. In the Silver Star Story Complete remake, this transformation was left out completely. Chalk it up to cliche overdose.
In both versions of Lunar: Eternal Blue, Zophar has multiple forms. When he finally appears on screen, he's this giantic Eldritch Abomination, but after he kiddnaps Lucia and takes Althena's power, that form turns into his fortress and what he turns into... saying he looks like a crossdresser would be an understatement. Despite how powerful the plot makes him out to be, the general opinion is that he is very easy, which even the game's characters seem to notice, but then he goes into his ultimate form, in which he is takes up a big chunk of the screen. In the original version, he still looks very femine, but in the remake the designers realized they freaked the player out enough and make him look more masculine. Both versions, you can't hurt him until Lucia frees herself. His first form was just a warmup really, his 2nd is going to a nightmare unless you've done a lot of level grinding. He has one more form after that, which you only fight with Hiro and Lucia. This form is, not supriousingly from the look of it (a chunk his his upper torso attached to his head in the original, and wierd disembodied beak in the remake) is a cakewalk even though you only have two characters (Zophar's 2nd form was meant to be the ultimate challange of the game, the 3rd one is practically a scripted fight).
Parodied hard in the Sega CD adaptation of Space Adventure Cobra. When a rather fragile sentient plant is confronted, it proceeds to laugh mockingly at Cobra before turning into a gigantic demon. Its speech is cut short at "My name is..." when Cobra blasts it, splattering it all over the room. Afterwards he notes, "Next time I'll just whack it upside the head with a newspaper."
Revelations: Persona does this to its Big Bad, Guido, when you defeat him. The associated dialogue is just too ridiculous, funny and Macekred to not include:
Guido: This can't be! I'm a God! I'm invincible! (pauses) Something's invading my body!!
Massacre's voice: Hahaha! Stupid human, I shall give you the power you desire!
Mary: What's happening?
Mark: What the heck!?
Nate: He was taken over by his own Persona!
Guido: Now I'm Super Guido!
From Persona 3, the Appraiser of Death first shows up in human form ( two of them, in fact.) When it is time to make a choice concerning the fate of the world, he claims that he will soon turn "into something unrecognizable" —sure enough, during the Dark Hour of the Promised Day, he becomes Nyx Avatar, the Shadow of the Death Arcana with four midnight-black wings and a grinning white mask.
In Persona 2: Innocent Sin Nyarlathotep first fights the party as Hitler, then transforms into a monstrosity that's way too hard considering he's just screwing with the party by that point. Later in Eternal Punishment he gets slightly more serious, fighting in his default form of the Moon Howler and once you beat on that enough...
Nyarlathotep: Fuhahahaha! This is! Splendid! You are the first to see this form! Die with my highest praise!(sic)
He turns into a tentacle monster instead of an angel. Considering the Tabletop RPG above and that he was behind Guido's transformation before being Macekred out, it becomes obvious that the Crawling Chaos just can't stay away from this trope.
Persona 4loves this trope. The first time is just before the fight with Nametame, who is possessed by Kunino-sagiri and turns into him. Then, the real killer does it, transforming into Ameno-sagiri, God Of Fog. If you get the True Ending, Izanami does it twice, first becoming her Goddess form, and then turning into Izanami-no-Okami when you use the Orb of Sight. On top of that, everyone's Shadow does this. There are eight major Shadows... so Persona 4 does this twelve times in total.
A minor, nonfantastic version of this occurs in Ace Attorney Investigations: Miles Edgeworth. A common theme in the series is for villains (especially the Big Bads) to take on different demeanors after you've exposed their true nature but before they're defeated, with one of the most dramatic being Matt Engarde in Phoenix Wright: Ace Attorney: Justice For All. However, Quercus Alba practically changes into a different person when you expose him as leader of the smuggling ring. He stands up, throws away his shawl, snaps his cane in two, and goes from cowering to smirking. And then shit gets serious.
Investigations 2 has Souta Sarushiro, who, upon being revealed as the mastermind behind the recent deaths, seems to change his makeup and demeanor for no reason other than to look awesome. And it works.
Finally, from the DLC case, there's Marlon Rimes, who, upon being accused, drinks a few gallons of water from his barrel, and throws it aside. Cue instant bulk-up, a large fish to carry out of nowhere, massive gold chains, and pirate hat to top it all off. And there is where you begin to understand how he got his rapping career.
The final boss of La-Mulana is Mother, who takes five forms: a large stone face, a white flying silhouette, a disturbing Virgin Mary look-a-like complete with what resembles Baby Jesus in her arms, a pair of eyes, and finally, a smaller but no less deadly version of her second form.
Subverted with great gusto in Dark Cloud 2: the true form of the terrible, Dark Emperor Griffon who has been erasing people and places out of existence is a cute anthropomorphic bunny (or "Moon Person" in the Dark Cloud world) no taller than Max. However, when he absorbs the power of the Sun, Moon, and Earth Atlamillia, he becomes a towering, muscular behemoth with enormous sapphire wings. During the battle with him, he can even rip off these wings to use as swords; as his power destabilizes, he grows even more muscular and his veins shine with the magic running through them. Defeat reverts him to his small, fluffy, and adorable lapin form.
The previous game in the series, Dark Cloud, played this one straight, more or less, but also pulled a bait and switch with it. The enormous, fat, oafish-looking Dark Genie, when destroyed, turns out to have been a mouse that got sealed in the urn with the Genie, and had absorbed some of its power. Then the REAL Dark Genie manifests itself as a tower of muscle with no lower body, arms that can punch up out of the floor, and killer magic beams. THEN, when you kill it in THIS form, it turns into its final form—a massive creature resembling Gospel from the Mega Man franchise.
Even a vertical shoot'em'up with space fighters can have this. In the final level of Tyrian Vykromod, the alien assassin who's been stalking the player for some time, seems to turn into a giant floating face - a pair of eyes, a nose and a mouth
World of Warcraft has several bosses that transform during battles; Illidan Stormrage shifts into and out of his fully demonic Metamorphosis form (as opposed to his normal half-night elf, half demon form); similarily Leotheras the Blind shifts between being a Blood Elf and a Demon (until eventually splititng into two separate forms); similarily several dragon bosses who start out in humanoid forms. Saidan Dathoran/Balnazaar and Baeren Westwind/Mal'ganis revert to their true demon forms mindway through battle, being previously disguised as humans.
Demonology Warlocks can learn the "Demon form" talent that transforms them into a demon for a short period, buffing their armour and spellcasting. In its later form, it also grants defensive bonuses that often make it overlap with Emergency Transformation.
A few other bosses go through consecutive transformations throughout the fight. Thus, Eldritch Abomination C'thun starts out as a giant eye, before turning into a huge bloated body with a lot of teeth and eyes. Even more prominent with Yogg'saron, whose first form is humanoid, and second form is... well... his title is "The Beast of a Thousand Maws" and it fits him to a T.
The Black Knight is a regular NPC you fight in a mounted duel after a short (but annoying) quest chain. Being an agent of the Lich King, he comes back zombified as the final boss of the Trials of the Champion instance, resurrecting the announcer who he force choked earlier as a ghoul. You kill him, but you can't loot him. Because he's back AGAIN, only this time, he's a skeleton. And then he summons about 10 ghouls. And then you kill him. And THEN he comes back as a ghost, even more powerful than the ten ghouls of the previous phase. Which you kill. He finally stays dead - until tomorrow, when you do the instance again!
In the Icecrown Citadel raid dungeon, we have Professor Putricide (who happens to be one massive Shout-Out to Professor Farnsworth ). He imbibes some of his own concoctions during the fight, causing him to become extremely muscular and grow a pair of tentacles from his back a la Doctor Octopus.
Putricide is very much a parody of this trope. During his two transformations (it's a three-phase fight) he stuns the raid with "Tear Gas," runs to his lab bench, grabs a potion and declares "Hmm... I don't feel a thing. Whaaa?? Where'd those come from?" first, and "Tastes like... Cherry! Oh my! Excuse me!" the second time.
Also during the fight one of the raiders has to do this and become a Mutated Abomination and eat the ooze around the room that the Professor throws around and forms puddles.
The trash (ladies of the night) before the Maiden of Virtue (see the irony?) transform into their true form (sucubbus or undead) when they hit half health.
In the Dragon Soul, the final raid of Cataclysm, Deathwing undergoes a transformation between his first and second encounter. At the end of the first, he crashes into the Maelstrom, and at the start of the second emerges from it as a hideous monstrosity, complete with molten tentacles and appendages clinging to the platforms and attacking the players.
Similarly, in Warcraft III demon hunters like Illidan and mountain kings like Muradin can transform into bigger and monstrous version of themselves.
In Golden Sun, after defeating Saturos and Menardi, they transform into the two-headed Fusion Dragon.
The sequel has Agatio and Karst being turned into a pair of dragons, and the final boss is also a dragon made out of people, just not evil dudes going One-Winged Angel. And now Golden Sun Dark Dawn has the new villainous pair being absorbed into a dark beast thing while trying to power up the morphed Volechek
Nearly everyone that gets possessed by Rhapthorne in Dragon Quest VIII pulls some version of this.
In Soul Calibur 3 Nightmare pulls this off at the end of his story, an imput from the player turns him into Night Terror, a glow-y Nightmare with wings (and the true final fight of the game).
In Soul Calibur 4 Nightmare actually has a COMBO that transforms him into Night Terror.
Zasalamel from 3. When he transforms into Abyss...yeah, good luck. Though he's nowhere near as hard as Night Terror.
City of Heroes has Romulus, who, after you beat him with only moderate difficulty in the third mission of his arc, merges with a Nictus in the fourth. His new self was one of the most challenging fights in the game at the time, and now can wipe a unprepared team.
Incarnate Content doesn't use this trope as often as you would think, but it does have several notable examples. The first is during the Minds of Mayhem Trial, where you fight Mother Mayhem in three different bodies before facing her true form, a humanoid psychic projection formed from dying neurons that seems to be falling apart.
Diabolique does this as well, though not during her trial. When you first face her in Dark Astoria, she looks almost identical to her Alternate Universe counterpart, Numina. After she kidnaps Praetor Duncan she reveals her "empowered" form as a towering, Stripperific Death Goddess.
Finally, Big Bad Tyrant skips the formalities and nukes his own city before absorbing the souls of it's dead to become a 30 foot tall (but otherwise identical) version of himself.
Panzer Dragoon Saga. In a single, long, psychedelic battle you first fight the five extreme forms of your own dragon, then Sestren - the final final boss, who changes into an even more horrible form.
''Video Game/Espgaluda 2's 4th stage boss starts small, then becomes gradually bigger (by assimilating mechanical accessories) as he's nearing his defeat (as seen here). Normally, in shmup games, many non-final bosses feature minor weapons or parts that are (optionally or not) destroyed or discarded, as the boss start using increasingly more difficult attacks, and this example comes as an unexpected, uncommon inversion
In Blue Dragon, Big Bad Nene merges with a giant eternal engine to supply himself with infinite magic power for the climactic showdown. And the REAL final boss, Destroy. When Deathroy (the little guy by Nene's side for the whole game) swallows Nene's soul, he reveals his true form as the monster that previously ended the world.
Nero Chaos in Tsukihime when he finally realizes that he is getting his ass kicked but refuses to run away eventually joins all the chaos left in his body into its ultimate destructive form, which isn't very well described except that it looks 'efficient.' The motion blurred picture given looks something like a worgen.
Dark Raven from Billy Hatcher and the Giant Egg turns into a giant shadowy crow, with white eyes, and (eventually) covered in sparks after he hatches the titular Giant Egg.
Played straight for a few bosses in Batman: Arkham Asylum, but subverted for the final battle; Joker uses Titan to turn himself into a 15-foot tall hulking monster... then spends most of the fight mocking you from the sidelines while you fight waves of his Mooks, just like always. To be fair to the guy, this wasn't his original plan: he actually wanted to force such a monstrous transformation upon Batman, then uses the drug on himself in an attempt to overcome Batman's Heroic Willpower to resist it.
Luminous Arc: the last boss turns into a beautiful white feathered serpent with lots of angel wings for its final form. The second game, Luminous Arc 2: does a similar thing as well except it resembles a fiery phoenix/giant plant.
In the fight against the Doctor, he starts off looking like himself. Upon defeat, he loses control of theRed Crystal and transforms into a muscular berserker. After this, he dissolves into a red mist, which transforms Misery (whom you fought right before the Doctor) into a monster and forces her to fight as his flunky, and he possesses the Core (which you also fought earlier in the game). So the final round of the Doctor's fight is against One Winged Angel versions of three prior bosses. And Sue.
The True Final Boss starts off as a humanoid, then transforms into a giant, freakishly-smiling head. When you beat that, the head grows eight more eyes. When you beat that, his shell partially crumbles and you can see moaning faces within.
Disgaea 2 plays this trope straight as an arrow with its final boss, having fake Zenon transform into a bizarre creature that's covered in masks.
Disgaea 3 plays it straight and lampshades it, when Aurum takes on an angelic-looking form, which is commented on as being cliche and boring.
Valvatorez of Disgaea 4 provides a heroic example, with his final attack transforming him into a humanoid dragon. Emizel, Fenrich and Desco likewise transform in their final attacks (Ironically enough, the latter is a Final Boss-in-training).
In Lego Indiana Jones 2, the end bosses of Raiders (Belloq), Crusade (Donovan), and part 2 of Crystul Skull (The Soviet Colonel) are all given a One-Winged Angel makeover, in forms that overlap with Rent-a-Zilla.
Alpha Prime has a strange case where the hero actually inflicts this upon the villain. The hero had previously heard that Glomar's heart reacts to the thoughts and personality of those who touch it, so he puts doubt into the villain's mind that it will not, in fact, give him power, but destroy him. When the villain touches the heart, it warps his body into a hideous abomination.
In the first game, the Saccubus turns out to be a projection of Dark Force.
Phantasy Star 4 has the Profound Darkness, which starts out monstrous, then gets more streamlined, and then turns into a naked lady a la 2's Mother Brain.
Phantasy Star Online has Vol Opt/Vol Opt Version 2, an insane computer with two forms; Dark Falz, which starts out as a strange three-legged THING, then turns into a floating spell-throwing machine, and then in the higher difficulties it turns into a much meaner floating thing with seriously unpleasant physical attacks and periodic invulnerability; and Olga Flow, who turns from an upside down six-legged warrior-thing into a two-legged warrior thing about the size of a large BUILDING.
C.A.R.D. Revolution has a double-barrelled final boss that turns from a girl with a sword-arm and monstrous chunks of mutated flesh that defines Body Horror into a thankfully less-messed-up creature vaguely reminiscent of a Delsaber.
Universe has a couple of versions of Magashi, and a double-barrelled Dark Falz with the added bonus of one of the coolest yet least feasible arenas of all time. Dark Falz reappears in Ambition of the Illuminus in a side mission.
Portable has Helga, Vivienne and Dark Falz AGAIN.
And finally Zero has Mother Trinity, who turns out to have been hosting Dark Falz.
Dawn of War has a few of these: Chaos Champions and Heroes can function as the host of the Blood Thirster, a Greater Daemon of Khorne, the Necron Lord can transform into the Nightbringer or the Deceiver and an Eldar is sacrificed to bring about the Avatar of Khaine, though the latter isn't a game mechanic and the Avatar is produced like any other unit. It is shown in a cutscene though, They summon Khaine from a Dark Reaper aspect warrior, to fight against the Blood Ravens in the last mission against the Eldar
And there's the chaos sorcerer Sindri Myr who does this during the intro for the final mission, screaming "bear witness to my ascension" while tapping into the power of the Maledictum in order to become a daemon prince. It doesn't help though; the Blood Ravens still kill him
A rare case where the user is on your side occurs near the end of Dawn of War II: Retribution, where during the Eldar campaign a dying Howling Banshee offers herself to the heroes to awaken an Avatar.
Possibly spoofed in the game The Dark Spire where you encounter a "One Winged Angel" in a circus exhibit.
Celestia Lindwurm, final boss of the shmup E Xceed 3rd: Jade Penetrate, transformed from a girl with wings into... a girl with larger bizarre-looking wings. Then Black Package came along, and she instead becomes a something like fifty-winged angel.
In most every Teenage Mutant Ninja Turtles game that came out after Secret of the Ooze, Shredder would turn into Super Shredder. In the SNES version Turtles in Time he would do this without you even fighting him as normal Shredder.
Ragna the Bloodedge from BlazBlue transforms into... some winged creature covered in shadow as part of his AstralHeat. Then reverts to his normal form for the winpose.
In Overlord II the final boss encounter is the Great Devourer. The Emperor, having gathered magic from across his kingdom, submerges himself in it and is transformed into a massive, glowing, zombie-spewing larva. Link.
Rise of the Kasai features four bosses(two are fought at the same time) who transform before the battle even begins because...who can blame them? The heroes just infiltratied their impenatrable fortresses and slaughtered their mook armies. Three transform into dragons, and the final one transforms into a hideous spider like monster with the animated corpses of its mooks fused to its legs still lashing out at the heroes.
Mr. Big of NARC is a fat man in a wheelchair who fires rockets while dozens of his henchmen dogpile you. After he is killed he comes back as a giant head on a floating platform whos flesh gets blown off to reveal a gaint metal skull.
Armon Ritter of Sin and Punishment: Star Successor has based most of his fighting style around this. He has three One-Winged Angel forms: The first is an enormous bat-looking protozoa capable of summoning missiles, floating balls of goop that attack you at both long and close range, and fighter jets. His second is an giant insectile seahorse that can box you in and play a game of deadly pong in that energy box with you in it. And his final (and hardest) form? Five killer whales. Scareeee. At least all of his forms have a Godzilla/Mothra-esque scream that accompanies every charge shot you throw at him.
Later on, it turns out that every single one of the Nebulox has a One Winged Angel form, as well as you. This makes for one of the most entertaining levels in the game, along with one of the most challenging bosses.
The final boss of Super Adventure Island transforms into a freaky pig-elephant-gargoyle creature after becoming a normal angel as you kill its normal form.
Mem Aleph, the Normal and Law path final boss of Shin Megami Tensei: Strange Journey, turns into a fetal being in a technicolor force field after you defeat her normal-looking form. This trick has been pulled in other games, notably Ōkami, but Mem Aleph's "empty" form makes up for it by being one of the hardest RPG bosses in existence.
It's not just her, either. The first four bosses come back in bigger, more powerful shapes in Fornax (their power is fully unhindered there); Morax evolves into Moloch, Mitra evolves into Mithras, Horkos evolves into Orcus, and Asura evolves into Asherah. Jimenez and Zelenin also enter their respective One-Winged Angel shapes as Awake and Soil Forms for Jimenez and Judge and Pillar Forms for Zelenin.
Digital Devil Saga's Hari-Hara has a reasonable first/first and a half form (it switches between them)...and then turns into a huge elemental core-creating thing.
In Shin Megami Tensei III: Nocturne, Kagutsuchi starts off as...well, as one person described it, a giant disco ball. His second phase is a giant face...that spends the rest of the fight firing off a super-powerful almighty spell every other turn.
Not just Kagutsuchi. Chiaki, upon rejecting the last scraps of her humanity and becoming Baal Avatar, becomes a literal One-Winged Angel. The other Reason bosses might also count upon their respective ascensions.
In Shin Megami Tensei IV both Merkabah and Lucifer have one of these, which is pretty impressive as neither of them were that pretty in the first place.
Altered Beast was built around this trope. The player(s) fight their way across a scrolling platform landscape until they have collected enough power orbs to transform into their final beastman form (with special attack!). Then, when they next run into the member of the brotherhood of evil (a bald man in a coloured robe) that inhabits that stage, he says "Welcome to your doom!" and transforms into a huge (and usually surreal) boss monster.
The arcade game installment of the Cadillacs and Dinosaurs franchise: stage boss Morgan (a hunchback with a sub-machine gun) transforms into Morgue (a pachycephalosaurus knock-off) when defeated. Final boss Dr. Fessenden does this twice: first into a Morgue-alike, then into a two headed tyrannosaurus shivat with Fessenden growing out of its belly.
In Wardner, the eponymous Final Boss takes a few licks in his evil wizard form, then transforms into a giant brown demon and starts spitting out a continuous maze of descending fireballs.
The normally body-less Big Bad of An Untitled Story takes a total of five formsnote four on Simple, starting from The Boss look-alike, a duo of flying rings, a walking mecha (and its head) and a disturbing pair of disturbing eyes.
Subverted in the Sam & Max Beyond Time And Space episode "What's New Beelzebub?" with The Soda Poppers. Their "demon forms" are just different outfits, two of which differed only in color.
Played with in No More Heroes 2: Desperate Struggle with Jasper Batt Jr. Played fairly straight for his second form, for which he injects himself with steroids and dons a themed superhero costume (not unlike Batman's), before unleashing hell on you. Outright spoofed with his ridiculous third form, for which he somehow turns into a massive flying mascot-like balloon version of his former self - with a good dose of Clipped Wing Angel for this battle is MUCH easier than the second phase.
Dot Hack GU: Azure Kite reveals a form called the "Azure Flame God" after the player defeats him in the first volume. The true Tri-Edge, fought at the end of volume 2, also has a transformation of this kind. Finally, in volume 3, Sakaki also brings one out. Interestingly, most of the heroes in the games (including the protagonist) have One-Winged Angel forms alternately called "Avatars" or "Epitaphs", which they use to combat AIDA, as well as the various aforementioned transformations. These Epitaphs are... not the safest of powers to use.
The final boss of Odium, Vasili Dobrovsky, bursts into the room as a human, provides some exposition while announcing that he's the only man immune to the Viral Transformation that turned everybody into monsters, and when he's outed as The Dragon, he proceeds to turn into a giant monster anyway.
That said, if you don't know that Sariel has a final form, you are likely to have blown all your bombs defeating its first form—and thus have no bombs, one life, and are fighting something with unknown attack patterns.
Happens to the T-1000 at the end of the SNES version of Terminator 2: Judgement Day. After a Ring Out Boss fight where you shoot it to knock it into the steel vat like in the movie, the T-1000 emerges from the vat as a giant man-shaped blob that spits molten steel at you.
The Scott Pilgrim Game parodies this in Gideon's first boss fight. His main mode of attacking is transforming into a statue of an actual One-Winged Angel.
Actually, the game plays the trope fairly straight as well. The first form of Gideon you face is Super Gideon, which is by itself a hulking, brutish form of him. Then he transforms into the aberration known as Gigadeon, which is a bona fide One-Winged Angel and references Kefka Palazzo's boss fight (see above). Subverted as the real final battle involves fighting a human-sized, robotic lookalike of Gideon wielding a pixelated katana.
In RosenkreuzStilette, Graf Michael Sepperin assumes his demon form once he Turns Red. Also, in the final battle, Iris' One-Winged Angel form is that of her own humble self with three pairs of golden seraph wings.
In Jak II there's a twist at the end where Kor transforms into the Metal Head Leader, a house-sized monster that can fly and shoot lasers out of its mouth.
Deus Ex has Big Bad Bob Page with drastically enhanced nano-augmentations hooked up to a large ultimate augmented machine-thingy. Though it just makes him immobile, and one walkthrough even points out he's every bit as threatening as you'd expect a naked man in a snowglobe to be.
War, the third horseman in Apocalypse, grows to 50 feet tall in his second form.
In Albion, the Cuain, leader of the Kenget Kamulos, gets this as a perk with the job. He can turn into the avatar of Kamulos, the god of war. You fight the current Cuain and it's one of the very few real boss fights in the game.
In AdventureQuest Worlds, after being called pathetic for hiding in his teddy bear disguise by Lord Krom Wrath, Deady insults him for his lack of style sense and complies with his desire to see his true form before pulling off his teddy bear head and revealing his true form to be a giant tentacled skull with white eyes with light purple pupils in its eye sockets that goes by the name of Urkor Malravenus.
Haunting Ground has Lorenzo who starts out at first as a creepy old man with paralyzed legs who can only crawl pathetically towards you. He then proceeds to turn into a much younger version of himself with seemingly superpowered punches. You defeat this version by pushing him into a pit of fire. He emerges from this in his final form: a flaming skeleton who can kill you in one hit.
GLaDOS from Portal, although it's more of inversion, with Forms of Morality, Anger, Curiosity and Cake being lost.
Bertrand from inFAMOUS 2. As a powerful Conduit, he possesses the ability to transform into a gigantic half-insectoid half-reptilian beast called the Behemoth. Trouble is, it's his only ability besides transforming other Conduits into monsters, and it usually leaves him out of control until he returns to human form. As such, he's reluctant to transform into the Behemoth even in an emergency.
Inverted in the case of the Beast: he spends most of the game as a rampaging lava-skinned monster, laying waste to cities and showing no desire to communicate... up until the climax, when he transforms into a relatively ordinary-looking human to explain a few things to Cole.
In the Wii shooter Conduit 2, John Adams (yes, THAT John Adams) infiltrates Atlantis, bringing several of his Trust soldiers in to fight you while he tries to snipe you from far away. After you damage him enough, you knock him back through a portal and end up in Agartha, the center of the Earth. There, Adams is no longer a chubby old man, but a giant, armored and horned alien. He laughs and comments on how it had been a while since he had been in his "true" form.
In the Wii version of A Boy and His Blob, the Emperor of Bloblonia begins as a single blobby mass on a giant throne who goes down in a single hit. He then flees back to Where It All Began, and his form as the True Final Boss is an immense, be-tentacled writhing black beast that can only be defeated with the power of Mecha-Blob.
Tanzra in Actraiser starts out as a teleporting devil head, then turns into a much tougher skeleton demon.
Doctor Tongue in Zombies Ate My Neighbors quaffs his mad scientist potions and becomes, first, a giant spider, then a giant floating head of himself - that fires tongues!
The final boss in 'Readytorumbleround2' is a hulked-out version of Michael Buffer.
In beatmania IIDX 19, STN (representing Wrath of the Seven Deadly Sins) starts off as a mech soldier. Once you defeat him and get the Demon Feathers from him, Levaslater and Rche, you unlock Neulakyussra, who represents the apocalypse. In its video, you see STN's armor crumble to reveal a white-haired man who then becomes a four-armed, three-headed being with the crests of the seven sins on each arm and head.
Darksiders has a heroic example. Main protagonist War begins the game as a gigantic flaming monstrosity, having been sent to play his part in The End of the World as We Know It. Getting Brought Down to Normal (or as close as a Horseman can get) is what tells him that something is not right. Later in the game he can return to his monster form at will once he gains enough Chaos and it serves as his Super Mode. In a Perspective Flip of this trope's usual applications some enemies have the power to shut off this form.
Inverted with the final boss, the Destroyer. He starts out as a huge dragon, but after beating him enough, he reverts back to his original Angel form, although as a Fallen Angel.
In the Tower Defense game Kingdom Rush, the Final Boss will turn into a huge firebreathing demon after he is "slain".
The arcade game Smash TV features a difficult boss named Scarface: an enormous green, well, face who spits swarms of bullets and ricocheting mines at you. You'll spend five to ten minutes (and about that many lives) grossly disfiguring him with ammunition until he finally dies and explodes, only for his skull to scream "NO WAY!" and blast you with death beams from his eye sockets.
In Solatorobo, when you come to finally fight against Bruno, who up until then had been a regular-looking Caninu, he's been transformed into a hulking monster by Lares. Although Bruno does state that it's his true form, it's debatable whether this is true, or if Lares is simply using him as a puppet.
Arfoire in Hyperdimension Neptunia. Her final form? A dragon. It seems rather typical, but the game was also made to make fun of all video games.
The boss of "Green Inferno", the 6th stage of Gradius Gaiden, takes on three different forms before going down.
Shin Mitchell and Shin Scourge from the Ultra sub-series count as well.
Ultimate Replica from 4 who grows wings...and horns...and a tail.
Zone of the Enders: The 2nd Runner features Anubis fusing with the super weapon Aumaan, becoming Aumaan Anubis which stripped the mech of most of its armor and barely held together by a small number of wires and gaining a white color scheme. In place of its wings were a massive set of hexagonal plates ripped from the arena. Its already absurd power is shot Up To Eleven thanks to being fused with a weapon capable of destroying the entire universe. Luckily, you get to fight it in Jehuty's own One-Winged Angel form it gets by absorbing regular Anubis's Metatron, which gives you the ability to one or two-hit kill any enemy and boss in the game.
OFF has Elsens, who become Burnt and start spouting fountains of a thick dark substance, possibly blood or smoke, from their heads, sometimes gaining huge claws in the process, Japhet, whose real phoenix form comes bursting out of his cat form which is an actual cat named Valerie, and The Batter, who turns into a.... thing. Or maybe he was one all along, we never know, but it's pretty freaky.
The game Mystic Heroes has Emperor Kang's son, Cyrus, transform into a ridiculously ugly (and poorly rendered) dark greenish monster for the real final battle. The fight is somewhat lengthy, though, so be sure to take your time on this one. in a circus exhibit.
In most every Teenage Mutant Ninja Turtles game that came out after Secret of the Ooze, Shredder would turn into Super Shredder. In the SNES version Turtles in Time he would do this without you even fighting him as normal Shredder.
In Sly 2: Band Of Thieves, Arpeggio attempts to hijack the resembled body of Clockwerk to become immortal... and to finally fly. However, Neyla betrays him and takes the body for herself, becoming Clock-La, and serving as the final boss. Considering that Clockwerk is powered by hatred, and Neyla's heart is deathly black, it's fitting.
In homage to the aforementioned Super Shredder, Double Dragon Neon's Big Bad, Skullmageddon, transforms into Giga Skullmageddon for the true final battle.