Arivzae, Voice of Finality

Flying, lifelink
When Arivzae enters the battlefield exile target permanent until Arivzae leaves the battlefield.
At the beginning of your upkeep you may pay {x}, were x is equal to the number of cards exiled
with Arivzae, if you do, exile another target
permanent until Arivzae leaves the battlefield, if
you can't, sacrifice Arivzae.