Ertai's Familiar is a rare Creature — Illusion card from Weatherlight (WTH), collector number 38. Current market price: $0.83, TCGplayer market prices via Scryfall.
Phasing (This phases in or out before you untap during each of your untap steps. While it's phased out, it's treated as though it doesn't exist.) When this creature phases out or leaves the battlefield, mill three cards. {U}: Until your next upkeep, this creature can't phase out.