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Crumble Undead is a spell that requires a magic level of 39 to cast and costs 2 air runes, 2 earth runes and 1 chaos rune. It has a max hit of 15. This spell can only be cast on undead enemies, such as skeletons, zombies, ghost and shades. Crumble Undead is a good way to train magic, as it costs less runes than Earth Bolt (same if you are wielding a staff of earth) and hits as hard as Earth Blast. On the downside, it can only be auto-casted with a Slayer Staff, Staff of the dead, or Void Knight Mace. It is highly recommended against Slash Bash - The boss of Zogre Flesh Eaters quest.
Trying to attack a non-undead NPC with Crumble Undead says " This spell only affects skeletons, zombies, ghosts and shades".