Toxic Sludge

Once Hardmode starts, ordinary underground slimes are swapped out for these, and that substitution is the whole danger. You spend all of pre-Hardmode learning that slimes are background noise, then walk into one of these with the same lack of attention. Around half its hits also poison you, which shuts off your regeneration exactly when you were counting on it, deep in a cave with your health drifting down between fights. It takes no poison damage in return.

Royal Gel makes it passive along with every other slime, and early Hardmode cavern trips get noticeably quieter with it equipped. Its spawn rate was also cut back in a recent update, so it is less of a constant presence than it used to be.

It has a face, which almost no hostile slime does, and a visible grin, which puts it on a very short list of enemies that smile at you while they do it.