Implosion

Card Text

Choose one of your units in the Space or Ground arena. Put that unit face down under one of your opponent's units in the same arena. (Your facedown unit card is discarded when the unit is discarded.) Your opponent may pay 4 force to discard the facedown unit card at any time. When the battle phase starts, do X damage to your opponent's unit, where X is equal to the build cost of your facedown unit card.