The Dungeon Master

Whenever an opponent casts a spell or activates an ability, you may target it. If you do, roll a d20.

1: Your opponent may copy targeted spell or ability.
2-10: Your opponent may draw 3 cards or create 3 treasures.
10-19: Counter target spell or ability unless your opponent pays an additional {2}.
20: Counter target spell or ability and place it face down on the battlefield under your control. It becomes a 10/10 Legendary creature named BBEG with deathtouch, trample, haste, and vigilance.