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 to inflict the Poisoned debuff.

Toxic Sludge is immune to the Poisoned debuff.

Trivia

 * Possibly a reference to the Pokémon Muk which is made of toxic sludge.