Tidebinder Mage is a rare Creature — Merfolk Wizard card from Duel Decks: Merfolk vs. Goblins (DDT), collector number 22. Current market price: $0.19, TCGplayer market prices via Scryfall.
When this creature enters, tap target red or green creature an opponent controls. That creature doesn't untap during its controller's untap step for as long as you control this creature.