Sliver Overlord is a rare Legendary Creature — Sliver Mutant card from Scourge (SCG), collector number 139. Current market price: $37.92 (foil $208.89), TCGplayer market prices via Scryfall.
{3}: Search your library for a Sliver card, reveal that card, put it into your hand, then shuffle. {3}: Gain control of target Sliver. (This effect lasts indefinitely.)