Enfeeble

Enfeeble is a curse-type combat spell available at level 73 Magic. It costs 8 earth, 8 water, and 1 soul rune to use, and yields 166 experience when cast successfully. If cast unsuccessfully, like all other spells, you will be unable to cast any spell for 20 seconds.

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