Champions of the Shoal is a rare Creature — Merfolk Soldier card from Lorwyn Eclipsed Promos (PECL), collector number 46p. Current market price: $0.61 (foil $0.65), TCGplayer market prices via Scryfall.
As an additional cost to cast this spell, behold a Merfolk and exile it. (Exile a Merfolk you control or a Merfolk card from your hand.) Whenever this creature enters or becomes tapped, tap up to one target creature and put a stun counter on it. When this creature leaves the battlefield, return the exiled card to its owner's hand.