Toxic Sludge

The Toxic Sludge is a Hardmode slime enemy that appears in the Underground layer in place of normal slime enemies. They deal a vastly increased damage from normal slimes, and they also have a 50% chance on hit to afflict the player with Poison. They drop gel like the other slimes.

Toxic Sludge is immune to the Poisoned debuff.

Trivia

 * This slime, along with the Pet Baby Slime are the only slimes with a face.
 * This slime, along with Pinky, are the only slimes without "slime" in their name.
 * Probably a reference to Pokemons Muk: a pokemon made of toxic sludge.