The amount of life you lose is determined by the permanent's mana value as it last existed on the battlefield.
2024-11-08
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.
2024-11-08
If a permanent on the battlefield has in its mana cost, X is 0 for the purpose of determining its mana value.
Destroy target creature or enchantment an opponent controls. You lose life equal to that permanent's mana value.
Feed the Swarm solves one of mono-black's oldest problems: enchantment removal. Black historically can't touch enchantments, so this two-mana sorcery is a genuine gap-filler, not just another creature kill spell.
That flexibility is the entire pitch. You're paying for the option to blow up a problematic Smothering Tithe, Rhystic Study, Ghostly Prison, or game-warping enchantment that your black deck otherwise can't answer. The creature-destroy mode is incidental upside, killing anything regardless of indestructible/hexproof-from-... well, no, it's targeted, but it ignores indestructible.
Wants it: any heavily black or mono-black deck — control, midrange, aristocrats, stax mirrors — where enchantment hate is scarce. It's near-mandatory in mono-black where alternatives don't exist.
Drawback: the life loss scales with mana value, so killing a big creature or expensive enchantment hurts. In life-total-as-resource decks that's fine; in aggressive 1v1 or low-life builds, hitting a 7-drop costs you a lot.
Skip it if you're multicolor with cleaner removal (Anguished Unmaking, Beast Within, generic green/white answers).
in its mana cost, X is 0 for the purpose of determining its mana value.
