A Boss Battle is typically a dramatic moment in a game's narrative. The challenge the sequence presents to the player often mirrors the Player Character's struggles. As the player learns the patterns, chooses their moment to strike and whittles down their foe's health, they feel the same sense of triumph that...why is the main character the one being controlled by the AI!?
This is a type of Plot Twist, by way of Unexpected Gameplay Change, and means of Painting the Fourth Wall where the player controls a creature that would be more typical for a boss monster and tries to defeat something more akin to a Player Character (especially the protagonist they've been playing as so far). Such sequences are often for narrative effect and usually wind up being a Hopeless Boss Fight or a Puzzle Boss (since the player must usually win by deploying their attack patterns in the same way an actual boss would, or business as usual will resume after they've had a chance to sweat a bit). Sometimes they come about as a result of the main character becoming a Tragic Monster, and serve to make it clear that they're the bad guy now. The certain hero corrupted into a being that the protagonists fight against can cause this.
To qualify as this trope, the player must at least suddenly find themselves controlling an antagonistic character, or a character which plays like a traditional boss enemy would (i.e. much larger, fixed attack patterns, etc). If a character is fighting another player character but they are still in a protagonist role, it is Fighting Your Friend and sometimes "I Know You're in There Somewhere" Fight.
A type of Leaning on the Fourth Wall. Subtrope of Villain Shoes. Compare Rogue Protagonist (where the PC becomes a villain, but leaves the player's control in the sequel), Dueling Player Characters, for when a game with multiple playable characters pits them against one another, and Mook Horror Show, for when the hero is portrayed as a terrifying force while fighting bad guys. May overlap with Asymmetric Multiplayer in games where one player is a "boss" character and the other players are a team working against them.
Note: This is a Spoilered Rotten trope, that means that EVERY SINGLE EXAMPLE on this list is a spoiler by default and most of them will be unmarked. This is your last warning, only proceed if you really believe you can handle this list.
- At the end of Shadow of the Colossus, the Player Character Wander merges with Dormin to become essentially a new Colossus and has to fight a troop of priests trying to reverse the transformation. This scene, while brief, provides a chilling insight into how the sixteen Colossi that Wander killed to get there must have felt while fighting a small, nimble, and very determined foe for their lives.
- Played With Star Wars: The Force Unleashed: In the game you control Starkiller, Darth Vader's Apprentice who starts as a Villain Protagonist but ends to make a HeelFace Turn during the game with Vader becoming one of the Final Bosses. But the first stage is a flashback in which you control Vader going to kill a Jedi Master that survived Order 66 and you adopted a child Starkiller that will be his Apprentice.
- Hotline Miami: In the level "Neighbors", the player character (Jacket) fights and kills Biker, a cleaver-wielding Bullfight Boss. Near the end of the game this fight gets revisited, except now you are the Biker and have to kill Jacket. Ironically, the proper way to complete this fight is not to act like a Bullfight Boss, but simply run up to Jacket and kill him with one swipe.
- Metal Gear Solid 4: Guns of the Patriots: The last section of chapter 4 drops the game's stealth mechanics in favour of having Snake pilot Metal Gear REX, a Humongous Mecha boss from Metal Gear Solid, for a mech v mech fight against Liquid Ocelot piloting a Metal Gear RAY.
- In the normal difficulty boss for Armed With Wings Culmination, you play as the series antagonist Vandheer Lorde fighting against the protagonist Lone Warrior.
- Part of the final stretch of BlazBlue: Central Fiction's storymode has the player take control of the main villain Susanoo in a battle against Es.
- Marvel vs. Capcom 3: in the Ultimate version, there's a "Galactus Mode" where you can play as the Final Boss Galactus to fight scores of the normally-playable characters.
- Purposely done in Power Rangers: Battle for the Grid. Many times during Story Mode, when the main heroes are supposed to lose against Lord Drakkon and his cohorts, the player actually takes control of the bad guys.
- Similarly, the Story Mode of Street Fighter V sometimes puts you in the control of one of the villains when their victory is what advances the plot. One good example is Bison vs. Chun-Li early on.
- Super Smash Bros. Ultimate's World of Light contains a segment where you play as Master Hand, the most prominent recurring boss in the series. Also doubles as an Ascended Glitch, since before this game the only way to play with it was with a glitch involving name entry in Super Smash Bros. Melee.
- Part way through Axiom Verge, Trace starts to hallucinate from a virus in his system, causing the level to become a twisted nightmare. When the player reaches the boss, the normal dialogue is inverted (with the boss speaking like Trace and trying to talk him down, and Trace adopting the shared catchphase of "DEMON, ATHETOS SAY KILL"). The player then controls the boss monster in a Hopeless Boss Fight from the boss's perspective (which ends with Trace keeling over and being dragged to safety by a drone).
- Mega Man ZX Advent: The premise is that the main protagonist Grey and Ashe can use their Biometal, Model A, to transform into bosses that they defeated. As well, there are other Mega Men (those with the power of Biometal) that our protagonist can fight; one of them is the protagonist of the previous game, Vent/Aile and their Model ZX. So you can invoke this by turning into a monstrous boss form to fight him/her (although it's not the most effective choice, but it can be fun).
- Nefarious is a 2D platformer where you play as the villain trying to kidnap the princess, and all major boss fights except for the Becky version of the Final Boss run with this role reversal by putting the player in control of some big robot that fight like some familiar video game bosses.
- Sonic Mania features a boss fight at the end of Hydrocity Zone's first act where the player controls one of Dr. Robotnik's flying machines, and has to suck Dr. Robotnik into the machine's propeller to defeat him. This fight is a reversal of the one from the same zone in Sonic the Hedgehog 3, where Dr. Robotnik was controlling the machine.
- Super Mario Odyssey: This is a core mechanic, since Mario's new hat Cappy allows him to control any enemy that it's thrown at—even bosses. Several bosses thus have to be defeated by taking direct control of either them or some other aspect of their abilities and guiding the boss into being harmed by their own attacks. This culminates in the finale, where the player takes control of Bowser himself, in order to escape the collapsing ruins of his hideout.
- Done humorously in Yooka-Laylee and the Impossible Lair. At the end of the titular gauntlet, Yooka and Laylee steal Capital B's mind-control device and use it on him. The player, now in control of Capital B, can win the "fight" by deliberately steering him into hazards.
- Near the end of The Missing: J.J Macfield and the Island of Memories, there's a sequence where J.J transforms into the Hairshrieker, the monster that keeps chasing her, forcing the player to control her as a Background Boss and defeat Emily, who's wielding a shotgun like a typical action game protagonist. This serves as a mindscrewy metaphor for...suicide ideation, gender dysphoria, and Anger Born of Worry amongst other things.
- In the Bad Ending of Breath Of Fire 4, Ryu and Fou Lu (both of whom the player's controlled at various points) fuse into a dragon, with the final boss fight being against the player's own party.
- In the final chapter of Live A Live, you can pick who you want your protagonist to be, including Oersted, who you've just seen be Driven to Villainy. Should you pick Oersted, or rather, Odio, the scenario will completely change. It's revealed that the bosses of the first seven chapters were all different incarnations of Odio, and you must perform a reverse Boss Rush, playing as each version of Odio and killing the heroes off.
- In Paper Mario: The Thousand-Year Door, Mario fights Doopliss and defeats him rather handily. Then, Doopliss pulls a Grand Theft Me, taking control of Mario's body—and his entire group of partners follow along, none the wiser—and Mario becomes a shadow due to not having an identity note . The rest of the chapter involves him partnering up with Vivian, one of the Shadow Sirens, and taking on Doopliss-as-Mario and all of Mario's former partners so Mario can get his identity back. Meanwhile, Vivian sees how friendly and helpful the real Mario-as-a-shadow actually is and performs a HeelFace Turn, defecting from the other bad guys, who were abusive to her, and joining Mario's side for good.
- Early on Tales of Eternia, the party meets a man claiming to be a trader at the entrace of the cave who warns them of a dangerous eggbear that killed his partner inside the cave. When the party passes through the cave, they do indeed run into the eggbear. While a threat to a normal person, the eggbear is little more than an Elite Mook to the party, and they defeat it handily. Reid, being a hunter by trade and not one to let anything go to waste, butchers the eggbear for its meat and fur. They soon move on, and the incident is basically forgotten. MUCH later on in the game, when Reid is undertaking a series of trials to earn the special powers and Infinity +1 Sword he needs to fight the big bad, one of which involves Reid's conciousness being sent back into the past into the bodies of others. He's eventually sent into the body of an eggbear inside a rather familiar-looking cave. The player controls the eggbear, which can easily defeat the smaller, weaker monsters in the cave. When you get to the eggbear's den, you find that its cubs have been slaughtered by a pair of poachers. You kill one when they attack you, and the other flees in terror. As you pursue the fleeing poacher, you eventually run into your party and get slaughtered by them. And just as an extra punch, Reid's conciousness remains in the eggbear's lifeless body for a bit before returning to the present, long enough for him to witness himself butcher the eggbear, this time from the eggbear's perspective.
- In Tales of Xillia 2, the bad end requires protagonist Ludger to turn against his party members and fight all eight of them at once, alone.
- The mid-way twist of Chrono Cross involves Lynx switching bodies with Serge, resulting in a Hopeless Boss Fight against your own party.
- Late in Xenosaga Episode III, Shion pulls a FaceHeel Turn and joins the red Testament, a.k.a. her long-thought dead boyfriend Kevin, out of love for him. The rest of the party is forced to fight them, though thankfully, she returns to the party after they're beaten and Allen (finally) confesses his love for her.
- In Lord of the Rings: The Third Age there's a game mode that combines this trope with Boss Rush. After each chapter you complete in the main game, you unlock a new "level" in this mode that lets you replay all of the boss battles and mandatory battles against Mooks.... only this time, you're playing as the bad guys, and you have to defeat your party to advance to the next fight, which can be quite difficult in battles where you only have a handful of orcs with limited abilities and health; battles you likely swept through with ease in the main game. Completing these challenges earns rewards for the main game.
- Played with in Kingdom Hearts: Chain of Memories - when "Reverse Rebirth" is unlocked, the player character is instead Riku - and you get to play as his Superpowered Evil Side Dark Riku - meaning you have access to his moves that he used against you during Sora's story, even Dark Aura... except you can't actually use it for most of the game because the computer gets to use it with a different sleight combination than you do.
- The Sandman's bonus scenario has you playing as the titular Big Bad of the game and carrying out his plot to make everyone sleep forever. Naturally, the final boss of the scenario is The Heroine Sophie, and you face her in the same final battle that you fought the Sandman in the main game.
- Worldof Warcraft has a few instances of this. Most particularly in the Legion expansion, you get to play the final boss fight from The Burning Crusade expansion: but this time as Illidan Stormrage, the boss. You are, of course, taken down by a raid of forty AI-controlled players with expansion-appropriate abilities. The developers took the opportunity to poke fun at the game's players, with the banter lines and an assessment of average player skill at the time which was rather more accurate than some would care to admit.
- Sadly exaggerated in R Type Final. This is what happens if the player pilot killed Nomemayer in Beautiful Erasure (this encounter can be triggered by destroying the red rod before killing the boss from previous stage), and tries to escape but you got infected by Bydo and time traveled into Route B final stage which is the before the beginning of the first stage. The player pilot is oblivious about that he had turned into a Bydo, however you have to Kill Them All of your fighter friends and R-9A Arrowhead as a reverse Big Bad. Its Justified because during gameplay the player was brainwashed by the Bydo infection right after the intro of route B final stage, and got snap out of it (but still, the pilot doesnt realized that he is permanently corrupted into a Bydo) after destroying R-9A, Downer Ending indeed...
- Heroes of Might and Magic generally puts the player in control of a hero leading an army, commanding the army as they do battle with other heroes' armies and the occasional boss. However, in the first mission of Kiril's campaign in the 6th game, Kiril succumbs to Demonic Possession by the demon Azkaal, and the player takes on the role of Azkaal as he single-handedly slaughters an army led by Kiril's ally Xana.
- In Hard West, the player can continue to play as Warren and his party or can optionally play as The Undertaker and a party of his own assembled from characters from some of the other scenarios.