Etrata, the Silencer is a rare Legendary Creature — Vampire Assassin card from Guilds of Ravnica (GRN), collector number 170. Current market price: $0.69 (foil $4.34), TCGplayer market prices via Scryfall.
Etrata can't be blocked. Whenever Etrata deals combat damage to a player, exile target creature that player controls and put a hit counter on that card. That player loses the game if they own three or more exiled cards with hit counters on them. Etrata's owner shuffles Etrata into their library.