Consuming Aberration

Creature — Horror

Consuming Aberration's power and toughness are each equal to the number of cards in your opponents' graveyards.

Whenever you cast a spell, each opponent reveals cards from the top of their library until they reveal a land card, then puts those cards into their graveyard.

The ability that defines Consuming Aberration's power and toughness works in all zones, not just the battlefield.
An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack without resolving.
If an opponent has no land cards in their library, all the cards from that library will be revealed and put into their graveyard.