The Dark Maze is a survival challenge mode that unlocks at Player Rank 20. Descend deeper and deeper into the maze while facing enemies of increasing difficulty without having your party's health restored between battles. If a party is defeated, you can continue onward with a different one until you've used up all of your heroes. Collecting lots of powerful heroes and refining your strategy will aid your survival.
Much like a Titan, a maze boss' health isn't restored after each battle. It may take several attempts to take one down, so just keep chipping away at its health! Minions, however, will respawn with full health each time you enter the maze.
You have three weeks to work your way down as many levels as possible before the maze reshuffles, thereby resetting your progress and changing up the enemies. Each reshuffle poses a fresh maze with different monsters of different element types to battle. The Dark Maze functions in the same way for everyone though, so don't hesitate to exchange tips with other players!