The Tongue

The Tongue is a debuff inflicted by the Wall of Flesh when one of the following happens:
 * You find your way behind the Wall of Flesh.
 * You attempt to leave The Underworld by conventional means.
 * However, when you teleport out of The Underworld, you will get instantly killed after reaching your destination.

The purpose of this debuff is to prevent the player from getting behind the Wall of Flesh, or getting trapped. Upon activation, it causes a tendril from the WoF to attach to you, dragging you in front of it, ignoring blocks in the way, and do a lot of damage.

From enemy
Sometimes on the Wall of Flesh's death, it will drop the Wall of Flesh Trophy, which features the tongue.