Rek'Sai, the Void Burrower

Allegiance - The Void (Whenever a creature with Allegiance - The Void enters the battlefield under your control, put a bounty counter on target creature.)
{1}{u}, {t}: Rek'Sai, the Void Burrower gains protection from creatures with a bounty counter
on it.
{1}{b}, {t}: Destroy target creature with a bounty counter on it.