Shadow Ghoul of the Labyrinth

You can discard this card; add 1 "Labyrinth Wall" card from your Deck to your hand. At the start of the Damage Step, if your opponent's monster battles, while you control a "Labyrinth Wall" card: You can banish this card from your GY; destroy that opponent's monster. You can only use each effect of "Shadow Ghoul of the Labyrinth" once per turn.