Grue

What this mod does is adding a new mob called “Grue”, whom spawns close to a player if they’re in an area where the light level is 2 or lower (simply standing still will only make more of them spawn). The Grue is immortal, and deals almost infinite damage (so there’s no way to survive it if you get hit). But there is one way to banish it, which is as simple as getting into a place where the light level is higher than 2, in which case the Grue bursts into a plume of smoke, eliminating the threat for now.