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)