Rek'Sai, Void Burrower

Indestructible

As long as your devotion to blue and black is less
than seven, Rek'Sai is burrowed.

While burrowed, Rek'Sai isn't a creature.

Creatures you control have "{t}Target player puts
the top X cards from their library into their graveyard, where X is this creature's toughness."