"To the true Ridge Racer who has cleared all 39 tours, we introduce these tours of maximum difficulty. The first MAX tour, “Opus 1,” is the ultimate 4-car battle of Class 1 machines. Only 1 in 200 is expected to clear this challenge. There are no prizes. Race for your honor!" (Opus 1)
The extra difficult Secret Levels that some games have. They exist largely so that the player can brag to his friends about how awesome he is for winning them. A subtrope of Secret Level, distinguished from its mother trope by the fact that the extreme difficulty is pretty much the only point of these levels. Examples of this trope are, naturally, almost always That One Level.
The idea appears to be that if you're good enough to get there, you're good enough for whatever the game decides to torture you with.
Can be set in Planet Heck, but not always. Won't necessarily have a sign at the beginning saying "Welcome to Hell!", but probably might as well.
For the boss version of this, check out Bonus Boss, or alternatively, True Final Boss.
Beware of unmarked spoilers.
Hell Temple in La-Mulana takes this trope about as far as it can go. 'This place is one that none should come to'. They aren't kidding.
Sacred Grounds/Blood Stained Sanctuary in Cave Story. It even has a "Welcome to hell!" sign. Of course, completing this level (including the True Final Boss) is the only way to get the good ending. There are absolutely NO save points inside it, AT ALL. Even before the Heavy Press or Ballos, the True Final Bosses. And if you saved at the last opportunity and want to get some Life Capsules, you can't, as you've just passed the only Point of No Return in the game: using that Save Point. Even still, the Save Point before that ( which, humorously, is the same Save Point) is right before the regular Sequential Final Bosses, meaning you'll have to fight them again.
The "Final/Last Cave (Hidden)" is also one of these, and is required to get to the aforementioned Sacred Grounds, including a boss fight not in the regular version: the Red Demon/Ogre that Arthur drove away.
The Legend of Zelda: Twilight Princess has The Cave of Ordeals in a similar vain to Heros's Cave. Each room has a different set of monsters, and deeper rooms require later items to advance. While the rooms generally don't get too tough, the final room contains THREE Darknuts (four in the second playthrough). Since they tend to bulk together, parry attacks are difficult to pull off without getting hurt, and bombs are limited and not easy available.
The Legend of Zelda: A Link Between Worlds has Treacherous Tower, Lorule's equivalent of the Tower of Hera. It has up to 50 floors of enemies you have to defeat in large groups, acting like the Savage Labyrinth and Cave of Ordeals from other Zelda games.
Once the plot has been concluded and the characters have all reached closure, the final, bonus chapter of Dark Chronicle is the Zelmite Mine, the longest dungeon in the game. It's so long, it even has two bosses. The enemies are the strongest, fastest, and most resilient —two good hits from a boulder-type foe can kill one of your characters before you can even retaliate, and they wear down your weapons faster than usual.
In Persona 3, beating the Reaper lets you explore Monad, which is filled with extremely high level enemies who can easily wipe an unprepared party. On the other hand, they give out scads of experience, which is nice since the final boss is pretty much That One Boss.
Persona 4Golden has the Hollow Forest, a dungeon available in January and only if you max out the Aeon S.Link before defeating the so-called Final Boss. It's filled with extremely vicious, powerful enemies with loads of resistances, which make the Very Definitely Final Dungeon enemies look like wimps. Worse, you enter this dungeon while on vacation, so you have none of your vast collection of items, weaponry, or equipment, and can only use what you find inside. Oh, and the boss of the dungeon? Immune to everything but Almighty.
A similar experience is had when exploring Meteor Falls after beating the Elite Four in Pokémon Emerald. You may find a hidden NPC, former champion Steven's Pokémon at level 75+
One can also find Pokémon Trainer Red atop Mt. Silver in Pokémon Gold and Silver and their remakes, Heart Gold and Soul Silver. At the time of release for both the original and the remakes, that trainer uses the highest level Pokémon of any NPC trainer in any of the main games (disregarding trainers in the earlier Battle Towers and Battle Frontiers, before all levels were reduced to 50, anyway)
Pokémon Black 2 and White 2 has Black Tower in Black City and White Treehollow in White Forest for their respectively-named games. note Although, you can change the city if you have a friend with the corresponding game These two areas are giant mazes where, as you progress through the ranks, progressively get larger and more complex with the Trainers inside progressively getting tougher, as if their Pokémon weren't at a high enough level, anyway. The kicker? You have to find one certain Trainer in the maze to even get to that floor's boss, and the only way to find out if they're the right one is to fight them. Then you have to find your way through the maze to a special room where you can find the floor's boss. The Medic is a rare sight, too. But even if you do find them, they can and willkick your ass with their powerful Pokémon. Considering the fact thatThe Computer Is a Cheating Bastard is everywhere, and there's a tough Bonus Boss at the end, getting a Shiny Gible or Dratini afterwards is a very pleasant reward for completing Black Tower or White Treehollow. Fortunately, there is no legendary ban there like in other battle facilities. So feel free to unleash your team of level 100 legendaries on them.
In Tales of Destiny the bonus dungeon is a tower 60 floors high and special requirements need to be hit in order to get the treasure of each floor. Only a few of the levels actually offer you hints about what you should do, through randomly appearing cryptic messages throughout the game proper. All this should come as no surprise to those who know the game this was based on, The Tower of Druaga, which was just as nasty—except that, in Tales of Destiny, just getting to the tower is a Guide Dang It.
Secret Level 3 in the original Descent. Possibly the hardest level in the entire series.
The Doom series presents two notable examples...
Final Doom had the secret level "Go 2 It" - absolutely masochistic number of monsters, including a LOT of unfortunately-placed Cyberdemons. Casual players will spend hours struggling through this level to absolutely no avail.
Master Levels for Doom II had "Bad Dream," a secret level in the file TEETH.WAD. While the solution to this level is actually quite simple, being confronted with dozens of Cyberdemons at once allows the level to live up to its name.
Many modpacks have level designers who take secret levels as an excuse to let their insane side get loose with no restraint whatsoever. This has a tendency to result in levels that can't be ended without finding secret passages, ungodly amounts of very hard monsters in small rooms, timed sequences that must be done with ridiculous speed to have any hope of passing them, and so on.
Doom 64 has Hectic: An unforgiving obstacle course that is in a deceptively small map. One room has you fighting four Elite Mooks on a narrow ledge with no cover & a death pit, Another room plunges you into a pit with both a crushing ceiling, limited safe zones and Elite Mooks who will fry you quickly with their plasma guns. The third is not quite so bad, being a room with elevator platforms and dart shooters on the walls. Complete this level and you are granted the Cheating Menu.
Marathon Infinity has the Vidmaster Challenge, a kind of bonus level of hell for each of the three Marathon games. The game designers took the hardest level from each of the games, and made them WORSE, and put them back to back. And to top it off, the level If I Had A Rocket Launcher..., already insanely hard in the original game, starts with you stripped of all your guns. You start that one with an arsenal composed in its' entirety of two shotgun shells, one rifle magazine and eight grenades. They also use this opportunity to introduce an entirely new type of enemy.
Mile High Club in Modern Warfare. There were originally supposed to be 3 NPCs helping you during this mission, but because they were removed, you're doing the work of FOUR PEOPLE.
Wolfenstein 3D: The Episode 3 secret level is pretty brutal, but brilliant fun too, while the Episode 4 secret level is practically a death trap unless you know the exact route to the exit (or are just plain crazy!).
The first secret level in Spear of Destiny is no push-over as far as standard-style levels go either. What makes it special is the presence of Mutants who normally don't show up until the stages late in the game and the map being filled almost to the limit with enemies (149 is the maximum a map may contain in the old Wolfenstein 3D engine). The map also tends to have you fighting in narrow corridors with many blind corners.
Aztec Complex in GoldenEye (1997). The enemies have boosted AI and amazing firepower (pretty much everyone has an M16 or a Moonraker Laser), and the environment is biased against you every step of the way. Killing the boss — who is certainly no slouch — automatically trips the alarm, sending every enemy you haven't killed yet barreling straight at you with a blazing passion for your blood AND allowing them to respawn after a short delay. And there is only one piece of body armor and no health pickups.
Rom Hacks of Yoshi's Island typically have normal gameplay difficulty equivalent to that of the Extra levels in the original game - but these games usually also have extra levels which crank it Up to 11. If you were good enough to play the hack to begin with, you'd better have a lot more where that came from - otherwise, it's time to break out the save states!
Rayman Origins has the Tricky Treasure levels. Each one features Rayman pursuing a sentient treasure box as it weaves its way through a level that is falling apart. These levels require precise knowledge of where and when to jump (or not to jump), often acquired through trial and error.
Beating all 10 Tricky Treasure levels unlocks the Land of the Livid Dead. A single misstep or twitch in the wrong direction is enough to end in a very painful death. Words just... do not do it justice. See for yourself.
Most of the Livid Dead Party music levels in Rayman Legends are 8-bit remixes of the previous 6 music levels. The originals require jumping and punching to the beat of the music, depending on whether a gap or an enemy is in your way as the level auto-scrolls past you. The 8-bit remix levels, true to their name, introduce graphical defects to the levels that range from simply annoying to full-on Interface Screw. As an added bonus, the 8-bit remix levels don't have any checkpoints - you have to do a perfect run of each level from start to finish.
The levels required to unlock The Kid are based on The Kid's home game.
The Dark World levels are the (already punishingly difficult) original levels with a few extra saw blades or other obstacles strategically placed to cause the most death.
The Warp Zone levels limit your life count to 3 in a game where your total death toll will reach the thousands across a hundred levels before you reach "The End".
Chapter 7: Cotton Alley, the bonus chapter unlocked after beating the game. Enjoy your pink, colourful, cheery, disco, sawblade-covered death!
Catherine, a game that's already hellishly difficult in its own right, later presents a set of four very long, even harder levels known as Babel. Each one requires that you reach the top before all the blocks fall away, similar to normal levels. However, these levels are all set on Hard. This means that the floor falls away really quick and you can't undo any mistakes. Make a slight booboo in your strategy that makes it impossible to advance? Tough nuts. Very few people on eitherplatform have actually beaten these levels, and they only get harder as they go. And the final level, Axis Mundi is flat out impossible to complete alone solo in Western regions, thanks to a bug in the game that renders it impossible to climb more than a few steps. Hope you can work two controllers or have a friend to help you.
The first PSP Ridge Racer game (or rather, the US version) gives us the MAX tours, unlocked after completing all main tours. These are the 7 most ball-bustingly hard tours in the entire game, with Rubber Band A.I. like you wouldn't believe. Their description sums them up nicely:
In Wipeout 3, you can unlock four untextured bonus tracks. The first three are fairly easy. The fourth is also fairly easy unless you play on the highest speed class, In that case, if you haven't been playing the game since it came out, forget it.
The Lakeside stage in Sega Rally Championship 1995. Besides being longer than the other three stages, you need to be first by the end of Mountain to access it, which in itself is hard if you're playing on an arcade cabinet with the difficulty on 'Arcade'. And the track is PAINFULLY thin and hitting a wall just SLIGHTLY will send you to about 30mph dispite being at around 70mph throughout most of the other tracks. Did I mention that the time limit only gives you about 2.5 seconds between 'Impossible Lap Time' and 'Time Over'?
Not all of Guitar Hero's bonus levels are necessarily harder than the regular ones, but some definitely are. "Jordan" from the second game and "Through the Fire and Flames" from the third are the most famous ones.
Rock Band 2 has Visions. Then there's the DLC. Plus it seems some music is being written specifically for the game on RBN. Eep. And "Through the Fire and Flames" is now available for Rock Band.
The Bemani series is fond of these.
Dance Dance Revolution: In most games from DDR MAX onwards, clearing the last stage with a grade of AA or higher nets you an extra stage, which is usually rated a 10, scrolls at at least 300 BPM, has the x1.5 speed and Reverse mods in effect, and you can only miss 4-5 times before you get a Game Over. Clear and AA that, and you get the One More Extra Stage, a slightly easier song on which breaking combo is an instant fail.
Newer DDR games change this up a bit more, swapping out the non-recovering lifebar for a Challenge Lifebar with a variable number of lives on it and having the One More Extra Stage be even more difficult than the Extra Stage. Yes, we're looking at you, Pluto Relinquish!
X2 takes this even further; meet certain requirements and you go to the Replicant-D-Action folder for the extra stage. There's a total of six songs, all of which are hard (Anti-Matter, New Decade and Possession are almost twice as hard as the other 3 songs!). And then there's Valkyrie Dimension...
Beatmania IIDX also has Extra Stage and One More Extra Stage songs, but of particular note is Mendes, the One More Extra Stage song from IIDX 15: DJ Troopers. If you can actually clear it on Another (the hardest normally available difficulty) on the console version, which itself is brutal, you unlock an even harder Black Another chart for it. See it here - the left side is Another, and the right side is Black Another.
Perfect Cherry Blossom, the seventh in the series and the second of the Windows series, goes one step further by also having an even harder Phantasm stage, which pits you against Yukari Yakumo, probably the toughest Bonus Boss of the entire series.
How brutal is she? Yukari, using her Boundary of Life and Death spellcard, fires every single type of bullet in the game at you simultaneously. Bullet Hell, indeed!
In Imperishable Night you can unlock Last Word spellcards, which can each take hundreds of attempts to defeat.
The second game in the series, Story of the Eastern Wonderland has a similarly difficult extra stage in Evil Eye Sigma (a flying demon tank piloted by the local first stage boss), which is absurdly hard for entirely different reasons, most notably due to the game's larger hitbox and lack of modern conveniences like focusing.
The arcade version of Gradius III has a couple of optional hidden levels accessed at the very end of the game by letting yourself be hit by one of Bacterian's otherwise easily-dodged attacks. Instead of costing you a life as you might expect, you will be whisked away to one of two levels modeled after the first levels of the original Gradius and Salamander/Life Force games. While these levels are not necessarily that much more brutal than the rest of the itself brutally-difficult game they're in, they still throw you a curveball in that all of your powerups, speed-ups included, are taken away upon entry to these levels. There are only a small handful of powerups at the beginning of each of these stages, which pretty much have to be used for speed-ups, therefore you usually just have only your standard gun to take you through the whole level. Should you get to the end of one of these levels, you are not rewarded in any way other than the small handful of points you may have received in getting through the level; you simply get placed back in the "main" game (with all your powerups taken from you again, just for good measure) to take another shot at Bacterian.
The Gorge in Death Smiles. The Mega Black Label upgrade adds the Ice Palace, which is a bit more beginner-friendly than the Gorge (especially if you're using Sakura).
While the main story of Fire Emblem: The Sacred Stones was criticized for being uncharacteristically easy, the bonus dungeon Lagdou Ruins started out considerably harder than the end of the main story, and by its last few floors was unabashedly sadistic. The fact that you had to complete ten floors in a row with no saves in between didn't help matters, nor did the fact that most of the characters had low magic resistance and the latter floors were full of long-range casters that could twoshot them from across the map.
The Golems of AmgarrakDLC for Dragon Age: Origins consists of a single level filled with the meanest enemies you encounter in the entirety of the official DA:O content. In fact, it seems to exist solely for the purpose of finding out whether you are a bad enough dude/chick to take on four freaking boss-level enemies (plus two minor bosses) at once, on your own. The answer? You aren't, trust us. Unless you figure out that it's actually a Puzzle Boss. Oh, and that encounter is considered second worst to the Final Boss of the level.
After clearing the final story boss in the old AD&D "Gold Box" game Pools of Darkness, you probably had a party of 40th-level adventurers who were all dripping with powerful magical items. At this point, you had the option to take Dave's Challenge: a small dungeon with no safe spots that's crawling with every monster you hated fighting in the main game, as well as a few resurrected bosses.
Balue's Tower in Klonoa. It regularly hands out extra lives in packages of about 9. You will need all of them.
Also the House of Fun and House of Horrors bonus levels in Klonoa 2. You will need to have your full measure of wind-bullet shooting, enemy-head jumping, ear-floating skill, and be able to perfectly chain them all together to get through them. One slip up, and it's a life lost.
The Cow Level in Diablo II, mainly due to their sheer number. On the other hand, by the time you reach it as either class, you're more than likely to have some devastating skill at your disposal that will bring them down by the dozens in an instant.
Sector Z in Iji. One hit point, full armor, and the enemies are Goddamned Bats to some degree.
Spyro: Year of the Dragon: While most of the Super Bonus Round challenges are relatively easy, the Yeti skateboarding challenge is absolutely BRUTAL, much harder than even the hardest challenge in the rest of the game. You have to hit practically every speed star, rocket, and blue crab, as well as do major tricks off every skateboarding ramp, and if you fall off the edge or crash even ONCE, there's almost no chance of you winning first place. Even if you do everything right, you'll probably win by the skin of your teeth.
Some of the earlier bonus missions in Mafia Wars were very difficult (at least not without spending premium currency), which often led to complaints from players. Zynga has since toned them down a bit.
Any bonus course in the Jet Moto series, especially Nebulous in the second game.
Most of the Driving Missions in Gran Turismo 4, especially the final one.
Boulder Dash has difficult intermissions before checkpoint levels that you may start on. They are individually Nintendo Hard, and while they don't cost a life if you fail them, you get kicked to the next level without a chance to retry it. The hardest is the second intermission, the "V-bonus level"◊ where you need to make a mad dash while vulnerable to fast-moving square guardians. On the other hand, the third intermission is unwinnable on PAL systems.
Crash Bandicoot 1996 had three kinds of bonus levels, reachable through collecting sets of tokens through the level. Tawna's bonus levels are essentially breathers, where the player can collect extra lives and save the game or get a passwords. Brio's bonus levels consist of much more challenging jumping puzzles, with bigger rewards to match. Cortex's bonus levels are the absolute worst, with absolutely devious platforming challenges. Sadly, only Cortex's levels are obligatory for 100% Completion, because beating them unlocks extra levels...but should you fail them, you'll have to restart the stage you came from for another chance, and one of the bonus levels happens to be located in the game's resident Scrappy Level, Sunset Vista.
Super Monkey Ball. To even reach the Extra stages, you must complete all of a difficulty's stages without continuing. Clear Expert Extra without continuing and you get to the Master stages. And if that wasn't enough, in Super Monkey Ball 2, clear those without using a continue and you get the Master Extra stages. Good luck pulling that off on Deluxe, where you can only reach the Master stages via Ultimate mode, where you have to play through all Beginner, Advanced & Extra stages(there's a save feature for the mode, but it's only a slight solace).
The Binding of Isaac: Sheol is a literal hell, with wickedly hard monsters and Satan as the final boss. Wrath of the Lamb adds The Cathedral, a harder inversion of Sheol, and if you beat that while carrying the Polaroid, then you go to the Chest, where there is a boss in every single room.
Gauntlet: Dark Legacy has one bonus stage per world, that consists of trying to nab 25 coins in a maze before time runs out. Your reward for beating a bonus stage is a secret character; the secret characters have stat alignments similar to the base characters (Medusa excells at Magic like the Wizard and Sorceress, for example), but with more intriguing physical designs and overall higher stats. The problem? Said bonus stages range from antsy to teeth-grindingly brutal. Some of the more egregious examples include: a two-in-one literal Bonus Level of Heaven and Hell where you're locked into unintuitive control physics not used anywhere else in the game, and you cannot go back for any coins; a deliberately-confusing psychedelic maze; a space station where you must use teleporters that don't always work perfectly, all in time limits that can generously be called 'fatalistic'. Your punishment for losing is to re-appear in the stage where you found the bonus entrance, but the bonus door will be gone, and you'll need to replay the stage for it to appear again. Considering some of these bonus doors appear very close to the end of lengthy stages, the player is wedged between quite the rock and hard place.
Droidquest, the Java port of Robot Odyssey, added a sixth level with even more insane puzzles than the Nintendo Hard fifth level. Originally, you could only get there after collecting a number of secret items in association with the original Developer's Room, but the latest version introduced a portal straight to the sixth level.
Plants vs. Zombies: The Bobsled Bonanza minigame, where you face almost nothing but Zombonis and Zombie Bobsled Teams, with 4 ice tracks laid down so that the bobsleds can start swarming immediately. Zombonis crush all your plants instantly and you'll use up Spikeweeds (their one weakness) as quickly as you put them down. The bobsleds themselves are a pack of 4 zombies which move fast on ice (helpfully provided by the Zombonis), will quickly overwhelm your peashooters, will spawn if there's so much as an inch of ice laid down, and are difficult to bring down without expensive bomb plants. You can only clear the ice with Jalapenos, which have a cripplingly slow recharge rate. The Imitater is almost a requirement for this level, or you simply won't have enough bombs to clear the level.
In the NES version of The Bugs Bunny Crazy Castle, you can grab a no-carrot sign that sends you to one of four special stages, all of which are far more difficult than even the hardest of the normal stages. Beating one gets you three extra lives, but losing one sends you back three levels.
Terraria has several examples, but the best is the Hardmode Dungeon. Unlike the rest of the world, the Dungeon doesn't enter Hardmode after the Wall of Flesh bites it; rather, you have to kill off Plantera before it happens. Once it does, though, watch out. If you enter, you can expect hordes of skeleton soldiers, skeleton mages, skeleton ninjas, SWAT skeletons, and Paladins with four digit HP and attacks that can outdamage Skeletron Prime.
Kuru Kuru Kururin has three bonus levels in story mode when you finish each of the other levels without getting hit. These aren't very difficult though. The real Brutal Bonus Levels are the 5 mini-levels you unlock in challenge mode by finishing each of the 50 normal challenge levels without getting hit which is quite a feat in and of itself.
From Devil May Cry 2 onwards, the Devil May Cry series has featured Bloody Palace, a Brutal Bonus Level Up to Eleven. There are always a minimum of 99 levels in which the player has to fight a ton of enemies repeatedly. You can't use health restores or anything else, you just have to pray that you avoid basically every attack, or that enemies drop a lot of health (which occurs rarely). Becomes doubly hard since you'll also have to fight bosses from stage to stage. Mitigated in that you have the choice of advancing 1 or 10 levels at a time, so if you are in bad shape you can advance only one level and hope to recover some health before tackling harder levels.
The Professor Layton games have Layton's Challenges, a collection of 15 post-game puzzles (typically five sets of three puzzles each) unlocked by completing certain objectives in the main game. All of them are much more difficult than anything you'll face in the main game, with at least one puzzle in each 3-puzzle set being a fiendishly difficult (not to mention frustrating) slide puzzle and/or an insanely hard(er) version of one of the main game's already brutally tough puzzles. To top it all off, the very last puzzle in every game is, without fail, a diabolically difficult slide puzzle. Last Specter makes it even worse by making the final puzzle two slide puzzles in one, with absolutely no hints for the downright evil second puzzle. Miracle Mask gives a slide puzzle for its second last puzzle, with the final puzzle being different from usual. It's supposed to be a harder version of what was faced in the Azran Chamber, but being it's just stepping on buttons, it's really not as hard as you would imagine.
Otka Island, too. A maze constructed of over 100 identical rooms, only a few places where there are multiple correct paths (the minimum number of rooms to pass through to get to the boss is 47; to get all of the treasure first, 103 including the backtracking from the dead-end paths that the other treasures are down.) Your usual best weapon against bosses, the elemental summons? The boss of this dungeon can use it against you. Or more accurately, the members of your own party that he possesses can use them against you.
NightSky has a reward for collecting all the bonus stars hidden throughout the game, a final chapter called "Slightly Nonsense," which features some real challenges that force you to battle and exploit the environment physics every step of the way. This chapter includes, among other things, a level where you can only get around by working the anti-gravity power on and off, trampolines, and surfaces where the friction and impulse physics are intentionally wonky.
Spider-Man 2, after finishing the main game, you can then buy "Fight Arena", which allows you to fight hordes of enemies, and eventually, bosses. One of the bosses is Calypso, who doesn't appear anywhere else in the game. The final round of "Boss Arena" is fighting all four bosses at once (That's Doc Ock, Shocker, Rhino, and Calypso). Have fun.
Disgaea games always have these; the post-game is where the real challenge is. Considering the games' Absurdly High Level Cap (9999; you'll need well under 100 to beat the main story), these are usually Level Grinding fests. Many will have you acquiring new party members from previous games in the series (or other NIS titles). The real Brutal Bonus Level is typically against Baal or Pringer X, bosses with devastating attacks and stats in the upper stratosphere.
Particular mention goes to Baal in Disgaea 4, who has a passive ability that will instantly your character as soon as you place them on the map provided they aren't strong enough to survive the damage.
Jedi Starfighter has the unlockable bonus missions "Jango Fett", "The Lone Gunship" and "Advanced Training", all of which are far harder than most of the levels that you'll find in the main game. "Jango Fett" has the player controlling the eponymous bounty hunter as he flies Slave I and takes on a smuggler's private fleet singlehandedly, "The Lone Gunship" has the player controlling a Republic gunship pilot taking on a Separatist army on Geonosis singlehandedly (noticing a pattern here?), and "Advanced Training" is a follow-up to Adi's earlier Forced Tutorial that forces her to master advanced techniques like sniping and chasing in the starfighter.
LIMBO's Brutal Bonus Level is unlocked after you find and squish 10 hidden eggs throughout the game (not all in one playthrough, thankfully). The level is not simply difficult in that it throws more deadly shit than usual at you, instead it takes the same minimalist, artsy approach that the rest of the game does. Everything is completely black, save for your character's little glowing eyes, bouncing up and down. You have to dodge giant blade traps and solve puzzles purely by sound.
The Fallout: New Vegas DLC Lonesome Road has the Courier's Mile, which appears after you launch the missile from the Ashton silo. The area is irradiated to hell and back, and is swarming with Deathclaws and Irradiated Marked Men, the latter of which are much tougher than normal Marked Men and regenerate their HP thanks to the radiation. This area is not required to complete the main quest, but there are two warheads here, which must be detonated as part of the Warhead Hunter achievement. Bring plenty of Stealth Boys, Rad-Away, and sniper/anti-materiel ammo. At the end, if you launch the nukes at NCR and/or Legion territory, you gain access to two more irradiated areas housing the Bonus Bosses Colonel Royez and Gaius Magnus, who have even more rapid HP regeneration in addition to heavy armor and maxed-out SPECIAL stats.
The bonus areas in the Mega Man Battle Network series are known for being a fair step up from the main story in difficulty. But the Hidden WWW Network and the Secret Area of MMBN2 and MMBN3 respectively are widely regarded as being the most brutal in the whole series. Both games have viruses and bosses that are incredibly nasty. You also cannot warp out of the dungeon if the going gets too tough for you, you must MANUALLY leave the dungeon from where you came.
Think you can just sneak around and get the few treasures that you access? Think again! Some of the best treasures in the game are found here are just in plain sight, but to get the goods you have to beat a specialized encounter with viruses that MUST ALL be destroyed in one attack or they will ALL respawn (and if you don't kill them fast enough, they will cause a very powerful, unblockable explosion that will quickly kill you if left unchecked). Many of these encounters require the use of specific Program Advance attacks, most of which you will not figure out on your own without an outside source or sheer trial-and-error telling you.
Another reason why they're so difficult is because of the fact that at this point in the game, you MUST have a streamlined battle chip folder to get anywhere there. Streamlined as in — your folder has to be able to delete the enemies ASAP and/or provide Megaman great defenses; using that alphabet soup folder that got you thru the main story with minimal fuss is no longer going to cut it here, the bosses and event he viruses here are simply too dangerous to fight with only 1~2 battle chips tops per turn.
Mega Man Unlimited has Yoku Man's stage. You know those infamous disappearing and reappearing blocks that have been the bane of many players' existence ever since the first Mega Man game? Well, Yoku Man is the mastermind behind those things, and his stage has them scattered throughout the entire level over bottomless pits and spike traps, along with disappearing and reappearing spikes, blocks that become enemies that chase you until they die, reality-distorting prisms that blind your vision of the terrain around you, and the entire second half is one giant maze that sends you back to the beginning if you go the wrong way.
Abmneshi The Prophecy has the Invisible Stage, which is selected by pressing left with the leftmost stage already selected; It contains Sirisai, a rather nasty Bonus Boss.
Dynasty Warriors: Gundam 3 has the DLC mission "The Return of the Legendary Dynasty Warrior Gundam". It's only playable on the hardest difficulty, there are two Ace Pilots in the field using Musha Gundam and Musha Gundam Mk-II, both being able to kill the sturdiest playable gundam with two light attacks and that will fight the player in Fortresses that must be taken (and while fighting in a fortress, the player will be attacked by a neverending rain of rockets). Onde both fortresses are taken, the player will eventually face a giant mobile suit along the way (wich is also able to KO your suit with one or two well-placed attacks). And once the enemy gauge is finally depleted, all the enemies leave the field and the player must head to the main hall, while the Knight Gundam awaits. However, as soon as you get near him, both Musha Gundams will join the boss! And ANY strike from any of them is strong enough to take half of your health gauge. And as a nice touch for this mission, the game is programmed to only have one kind of item appear during the whole stage: the temporary 30% defense boost. That's right: absolutely no health recovering items will spawn during the mission.
Dungeons of Dredmor has a surprisingly easy to access bonus level: Diggle Hell. Mistype a wizardland code and enter the red, glowy portal and get ready to meet every single variety of diggle in the game, from the lowliest to the harbingers and even some exclusive to the place, and get dogpiled by them. It also holds a Bonus Boss, for those who like their unfairness with a dash of mercilessness.