Serum Sovereign is a rare Creature — Phyrexian Sphinx card from Phyrexia: All Will Be One (ONE), collector number 405. Current market price: $0.28 (foil $0.32), TCGplayer market prices via Scryfall.
Flying Whenever you cast a noncreature spell, put an oil counter on this creature. {U}, Remove an oil counter from this creature: Draw a card, then scry 2.