You can only control 1 "Stoic Challenge". Equip only to a face-up Xyz Monster that has Xyz Material. It gains 600 ATK for each Xyz Material attached to a monster you control, and any battle damage your opponent takes from battles involving it and their monster is doubled, but its effects cannot be activated. During your opponent's End Phase: Send this card to the Graveyard. When this card leaves the field, destroy the equipped monster.