Roshan, Hidden Magister is a uncommon Legendary Creature — Human Assassin card from Assassin's Creed (ACR), collector number 133. Current market price: $0.27 (foil $0.35), TCGplayer market prices via Scryfall.
Other creatures you control are Assassins in addition to their other types. The same is true for creature spells you control and creature cards you own that aren't on the battlefield. Face-down creatures you control have menace. Whenever a permanent you control is turned face up, you draw a card and you lose 1 life.