Steam

Probably my proudest is Crypt of the Necrodancer's Vow Down. The game is already very difficult, but Vow Down requires beating the game in all-zones mode (rogue-like mode rather than the rogue-lite campaign) with one of the more difficult new-game+ characters, Monk. The basic game is a turn-based, grid-based RPG, with the gimmick being that you have to preform all actions to the beat, while the monsters have their turn between beats. Vow Down adds the catch that any gold, be it dropped by monsters or found in the world, kills you instantly and ends your run. Even ignoring the inherant urge to pick up dropped loot, this means you have to carefully position enemies as you kill them so as not to box yourself in with deadly loot, all while only having a fraction of a second to think per turn.

This game also has what is probably among the most difficult (while still being fair) achivememts on Steam: Beating the game with the final New Game+ character, Coda, which has the debuffs of every other character combined. According to Steam, 0.3% of players have this achivement, meaning roughly 0.3% of players have cheated in achievements.