Rootwater Thief is a rare Creature — Merfolk Rogue card from Nemesis (NEM), collector number 40. Current market price: $2.53 (foil $41.44), TCGplayer market prices via Scryfall.
{U}: This creature gains flying until end of turn. Whenever this creature deals combat damage to a player, you may pay {2}. If you do, search that player's library for a card and exile it, then the player shuffles.