Heretical Hellbeast

Cocytus (Cocytus cards may only be put into decks through effects from Prince of Cocytus. If your library
were to be replaced, put all Cocytus cards back into the Cocytus Deck.)
Flying
Creatures your opponent control cannot attack you
as long as this creature is untapped.
When this creature enters the battlefield, it deals X damage to you, where X is the number of other creatures in play. Then destroy all other creatures.