Ghouls

Ghouls are Hardmode Underground Desert and Desert enemies. They follow the Fighter AI (behaving similar to Zombies). Ordinary Ghouls spawn inside the normal Desert, the Tainted Ghoul inside a Crimson desert, the Vile Ghoul inside a Corrupted desert and the Dreamer Ghoul inside a Hallowed Desert.

Trivia

 * While ghouls are a common enemy in many games, it is fitting that the Terraria version appears in desert biomes. The ghoul's origin can be traced back to Arabic stories of monsters that feed on the flesh of desert travelers, and desecrating the tombs of the buried.
 * It appears that the corruption/crimson and hallowed versions have a liquid dripping down their chin while the normal version doesn't.

History
Goules