Demon Heart

The Demon Heart is an Expert Mode and Master Mode consumable item from the Treasure Bag dropped by the Wall of Flesh. It permanently grants the player an extra accessory slot, for use only within Expert-mode and Master-mode worlds. This effect does not stack and may only be used once per player. If a player opens the Wall of Flesh's Treasure Bag after they have already consumed a Demon Heart, another one will not be obtained.

When a player who consumed a Demon Heart is in a normal-mode world (a non-Expert/Master world), the extra accessory slot is disabled, removing any functional or display effects from items equipped there, and darkening the slot itself. Players in normal worlds are also prevented from equipping items in the sixth or seventh slot, though an equipped item in the slot may still be removed.

Trivia

 * The Demon Heart is one of the few Expert-mode items that cannot be equipped in any accessory or equipment slots, the others being the Bone Glove and the three parts needed to craft the Mechanical Cart.
 * It is also the only consumable Expert/Master-exclusive item.