The amount of life you lose is determined by the permanent's mana value as it last existed on the battlefield.
08-11-2024
wotc
If a permanent on the battlefield has {X} in its mana cost, X is 0 for the purpose of determining its mana value.
08-11-2024
wotc
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.