Crypt of the NecroDancer.

There are three big challenge characters in the base game:

• Aria can only use the starting dagger, no other weapons. She has only one hit point. And she dies if the player ever misses a beat.
• Monk dies if he picks up gold. All enemies drop gold, even ones that normally wouldn’t, which turns the game into a routing puzzle where you must never step on squares that an enemy previously died on.
• Bolt plays the whole game at double tempo.

Once you have beaten these three challenge characters, plus the other six easier ones, your next task is All Chars Mode. Beat the game nine times in a row, once with each character. If you die, you must start the whole marathon over.

Beating that unlocks the tenth character, Coda. Coda combines the restrictions of Aria, Monk, and Bolt all at once.

And if you can do that, the final achievement is Lowest of the Low, which requires you to beat All Chars Mode without collecting any items.

The DLC adds a few more hard characters, and another achievement for an extended 13 Character Mode, but they aren’t considered to be as hard as Coda or Lowest of the Low. A single digit number of players have stacked the challenges for Coda low% and 13chars low%.