Mari, the Killing Quill is a rare Legendary Creature — Vampire Assassin card from Outlaws of Thunder Junction Commander (OTC), collector number 138. Current market price: $6.03, TCGplayer market prices via Scryfall.
Whenever a creature an opponent controls dies, exile it with a hit counter on it. Assassins, Mercenaries, and Rogues you control have deathtouch and "Whenever this creature deals combat damage to a player, you may remove a hit counter from a card that player owns in exile. If you do, draw a card and create two Treasure tokens."