Toxic Sludge

The Toxic Sludge is a Hardmode slime enemy that appears in the Underground in place of ordinary slimes. It deals significantly increased damage and has a chance of inflicting the Poisoned debuff, which it is also immune to.

Trivia

 * The Toxic Sludge is one of the few enemies in Terraria to have a visible smile, the others being the Clown, Snow Balla, and Snowman Gangsta.
 * Disregarding the Gastropod, it is the only enemy slime on which a face is visible. (Baby Slime minions, the Slime Prince, and Slime Princess have faces, but do not damage the player.)