Netherbound Atrocity

If Netherbound Atrocity would enter the battlefield, exile it instead unless you exile
3 or more creature cards from among graveyards.
{6}{b}{b}{b}: Monstrosity X, where X is the number of creatures exiled with Netherbound Atrocity.
When Netherbound Atrocity dies, if it was monstrous, return all creatures exiled with it to
the battlefield under their owner's control.