Toxic Sludge

The Toxic Sludge is a Hardmode Slime enemy that appears in the Underground in place of ordinary Slimes. They deal significantly increased damage and have a 50% chance of inflicting the Poisoned debuff, which they are 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.)
 * The Toxic Sludge seems to be an adult version of the Baby Slime summoned by the Slime Staff, given its similar colors and face.

History
Gadoue toxique Токсичная муть 毒泥