Spread of Carnage

Whenever Spread of Carnage enters the battlefield or you pay its cumulative upkeep cost, you may pay X life. If you do, each opponent sacrifices X permanents. If they control none, they lose that much life instead.
Cumulative upkeep--{b}, Pay X life. (If the value of X was determined when this enchantment entered the battlefield, its cumulative upkeep cost is that value. If not, you decide it the first time it is paid.)