Etrata, the Silencer is a rare Legendary Creature — Vampire Assassin card from GRN Guild Kit (GK1), collector number 1. Current market price: — (foil $1.23), 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.