The player is has to work in collaboration with this playing partner to clear levels. They can only clear levels if they stand at the final checkpoint together - holding a mythical artifact that gives then power, but also corrupts them slowly. Every move they make uses up some of their strength. When that's depleted the player re-spawns at a checkpoint on the level. If a player holds the artifact for too long, then they will start to lose sanity and die upon long exposure - that means both player would fail that level.

Players have to work and help each other - even if one is actually dying, both actually lose.