Regal Sliver is a rare Creature — Sliver card from Commander Masters (CMM), collector number 757. Current market price: $0.85 (foil $3.79), TCGplayer market prices via Scryfall.
Sliver creatures you control have "When this creature enters, Slivers you control get +1/+1 until end of turn if you're the monarch. Otherwise, you become the monarch."