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 fewer runes than Earth Bolt (same if you are wielding a staff of earth) and hits as hard as Earth Blast.
The spell can only be autocast by slayer's staff, slayer's staff (e), staff of the dead, toxic staff of the dead, staff of light, staff of balance, or void knight mace.
It is highly recommended against Vorkath, as the spell instantly kills the Zombified Spawn it summons during the fight.
It is also the best method for easily killing the tree spirit in the Lost City quest from a safespot.
Trying to attack a non-undead NPC with Crumble Undead says "This spell only affects skeletons, zombies, ghosts and shades".
The spell can always be paired with the salve amulet (ei), increasing its max hit to 18.