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Vulnerability is a spell in the standard spellbook that increases the target's damage received by 10% for 1 minute.
Before the Evolution of Combat, Enfeeble and Stagger (formerly called Stun) was used along side with Vulnerability for training Magic in metal armour because the spell will always miss with a low enough Magic attack bonus (-65). It could be re-cast indefinitely in this manner, resulting in quick experience. However, this spell no longer gives experience for missing, and is therefore not recommended for training Magic.