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gamerule doMobSpawning false.tick.mcfunction to immediately remove any non-zombie entities that appear.zombie, zombie_villager, husk, drowned, zombified_piglin) have PersistenceRequired:1b to prevent despawning.
MC: Zombified turns your Minecraft world into a zombie apocalypse! Only zombies will spawn, even in the Nether, replacing all other hostile mobs.