Rek'sai, the Void Burrower

Defender, trample, hexproof.
Tap an untapped creature you control: Rek'sai can attack as though she didn't have defender.
{1} Rek'sai cannot be blocked until end of turn.
Sacrifice X other creatures: Rek'sai deals X damage
to any target, where X is equal to Rek'sai and all creature's power combined. If a creature were to
die this way, exile it instead. This damage cannot
be prevented, but can be countered.