Ghost Mourner & Moonlit Chill

“If your opponent Special Summons a monster(s) face-up (except during the Damage Step): You can discard this card, then target 1 of those face-up monsters; negate its effects until the end of this turn, also if that face-up monster leaves the field this turn, its controller takes damage equal to its original ATK. You can only use this effect of "Ghost Mourner & Moonlit Chill" once per turn.”