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 this 'de-buff' 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, 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.