Sliver Overlord is a mythic Legendary Creature — Sliver Mutant card from Special Guests (SPG), collector number 128. Current market price: $29.86 (foil $41.62), 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.)