Siora of Seven Scripts

{u}, {t}: Target player mills one card. (They put
the top card of their library into their graveyard.)
{1}{b}, Pay 2 life: Untap Siora of Seven Scripts.
{4}{u/b}, Exile Siora: Create an X/X black Demon
creature token, where X is the number of cards
target opponent has milled this turn.