Merlin, the Wizard of Distress (world-2016-006) [Promo Cards]

[Stealth] When your opponent discards a card while you control a darkness magic stone. (While this card is in your chant-standby area, you may play it without paying its cost when its [Stealth] condition is met.) When this card enters your field from a chant-standby area, target resonator your opponent controls gain [-700/-700] until end of turn.