Toxic Sludge

The Toxic Sludge is a Hardmode Slime enemy that appears in the Underground layer 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

 * This Slime, along with the Baby Slime that is summoned via the Slime Staff, are the only two slimes that bear any kind of facial resemblance.