Marchesa's Emissary is a common Creature — Human Rogue card from Conspiracy (CNS), collector number 21. Current market price: $0.18 (foil $0.67), TCGplayer market prices via Scryfall.
Hexproof (This creature can't be the target of spells or abilities your opponents control.) Dethrone (Whenever this creature attacks the player with the most life or tied for most life, put a +1/+1 counter on it.)