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

Feed the Swarm

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

Foundations

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

The List

Outlaws of Thunder Junction Commander

The Lost Caverns of Ixalan Commander

If a permanent on the battlefield has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

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.