Feed the Swarm
Sorcery
Destroy target creature or enchantment an opponent controls. You lose life equal to that permanent's mana value.
"On vile wings and bloody wind, the swarm will rise." —Skyclave inscription
Illustrated by
Outlaws of Thunder Junction Commander#134CommonEnglishLegality
Rulings
Nov 8, 2024
The amount of life you lose is determined by the permanent's mana value as it last existed on the battlefield.
Nov 8, 2024
If a permanent on the battlefield has 
in its mana cost, X is 0 for the purpose of determining its mana value.
Nov 8, 2024
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.