Akasha, Queen of the Damned

Etrata, the Silencer 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.
(Etrata, the Silencer)