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The Zero Isle islands in PMD. There's 4 of them (Not including the center), here's their descriptions:
In order of difficulty:
Zero Isle North: 75 floors, Pokemon up to level 90, you gain no exp.
Zero Isle East: Player level is set to 1, 40 floors, can only take 16 items with you.
Zero Isle West: Level reset to 1, no items are allowed to be taken, 40 floors
Zero Isle South: Player level set to 1, Can not take any items or money, Pokemon as high as level 98, 99 floors, only one may enter.
And the mother of them all, Destiny tower, contains: Only one may enter, 99 floors, pokemon levels up to 99. This may seem feasable but there is one thing that makes the dungeon a nightmare: Grudge Traps. Grudge traps, when stepped on unknowingly, will summon a fuck ton of Pokemon next to you, all with the grudge status. Knock one out and, you guessed it, the move you used to kill it will have its PP set to ZERO. Combine this with level 99 pokemon all attacking you in the same turn, hazards such as pitfall traps which send you down a floor, traps that make important items unusable, pokemon traps that turn rare items in the room into more pokemon, items that turn food into sludgy toxic messes, and even traps that lower you movement speed, a death sentence in the PMD games as it means foes can attack you TWICE in one turn.
It's pretty hard to top