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, it is NOT called upon if you teleport out of The Underworld; if that happens, you die a quick death.

The exact nature of how the debuff works, or why it was created in the first place is unknown, as it is pretty apparent when it begins to affect you. Upon activation, it causes a tendril from the WoF to attach to you, drag you in front of it's gaping maw, and do a lot of damage to you. It is best to avoid it in the boss fight.