I figured that this theme would attract tons of endless survival games, and decided that I wanted something different. So my game needs to have a clear end goal while still having something inevitable to delay. The inevitable thing can't be failure (since otherwise, winning would make it not inevitable) but still needs to be bad. I struggled to come up with an idea, but after finding these guidelines I got something.

My idea: you're fighting a boss. There's a curse meter that slowly fills up, and fills up faster when you're in the boss' aura of darkness. When the meter is full, it empties, and one of your special abilities is permanently weakened or disabled. If you get cursed too much, you die, but it's possible to beat the boss before that happens.