The Abyss

Every upkeep it destroys a nonartifact creature the active player controls, of their choice, with regeneration explicitly shut off, which makes it less a removal spell than a tax on existing. Two clauses do the balancing work. The first is "their choice": the controller of the enchantment is not picking the target, so the card cannot be aimed, only deployed. That asymmetry is what keeps black from turning it into a Moat-style hard lock; the opponent always feeds it the worst creature and saves the best, which means the card grinds rather than wins. The second is the destroy clause: because it destroys rather than sacrifices, it slides off anything that cannot be destroyed and would respect regeneration shields in every case except this one, where the "can't be regenerated" rider closes that escape. The artifact exemption completes the prison: build around artifact creatures and the lock only applies to your opponent. The World subtype is the structural pressure valve that lets the design afford such severity: World Enchantments do not coexist, and any new copy displaces the old, so a rule change this brutal could never stack or sit alongside a rival. Reserved-list status and the scarcity of the original printing have kept it more talked-about than tabled, but the construction states plainly what a World Enchantment was meant to be: a global rule change severe enough that the game needed a mechanic to guarantee only one could ever be in force.