Vulnerability

Vulnerability is a curse-type combat spell available at level 66 Magic. It costs 5 earth, 5 water, and 1 soul rune to use, and yields 152 experience when cast successfully. If cast unsuccessfully, like all other spells, you will be unable to cast any spell for 20 seconds.

Vulnerability has the effect of temporarily decreasing your opponent's Defense level by up to 10%. However, the spell cannot be cast on the target if their Defense is already reduced, even slightly. As a result, the player cannot use both Vulnerability and Curse on the opponent to stack the Defense stat reduction.