"All it took was a little Time. And all for this, to get beyond any one universe, to find Division, to get revenge on those who imprisoned us so long ago. To take them all."

Feed the Swarm

Destroy target creature or enchantment an opponent controls. You lose life equal to that permanent's mana value.

The spineworms' venom contains a powerful anesthetic, allowing victims to watch in peace as their bodies are stripped to the bone.

Duskmourn: House of Horror Commander

"On vile wings and bloody wind, the swarm will rise."
—Skyclave inscription

The List

Outlaws of Thunder Junction Commander

The Lost Caverns of Ixalan Commander

Doctor Who

If a permanent on the battlefield has {X} in its mana cost, X is considered to be 0.

If the target permanent is an illegal target by the time Feed the Swarm tries to resolve, the spell doesn't resolve. You don't lose any life. If the target is legal but not destroyed (most likely because it has indestructible), you do lose life.

The amount of life you lose is determined by the permanent's mana value as it last existed on the battlefield.