The Tongue

The Tongue is a specific debuff called upon 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 (the tendrils can drag you over 100 blocks, by means of summoning WoF, dying, then quickly getting back to hell and running towards it from behind), ignoring tiles underway, and do a lot of damage to you, especially when engaged pre-hardmode.

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