Nether Ritual

At the beginning of your end step, sacrifice a creature and put a charge counter on Nether Ritual. If you can't, shuffle it to your library.
If Nether Ritual has ten or more charge counters on it, sacrifice it. If you do, create a legendary 10/10 black Avatar creature token named Kirganath the Tormentor with flying, hexproof, and "Whenever this creature attacks, defending player sacrifices all creatures."