Crumble Undead is a standard spell requiring 39 Magic 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, ghosts and shades.
Crumble undead can only be autocast by a skull sceptre (i), 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. In particular, skull sceptre (i) is the only free-to-play weapon that allows to autocast this spell.
Autocasting Crumble Undead costs fewer runes than Earth Bolt (unless a staff of earth or a Dust Battlestaff are equipped) and hits as hard as Earth Blast.
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.