Does the 10% affect your damage, or does it only boost the accuracy of spells?

Because it doesn't boost your Magic level in your skills tab, it probably doesn't boost max hit. However, it does lower your opponents magic level while raising your own accuracy, meaning it's probably much better to do use than Mystic Might.

Leech curse prayers are better than eagle eye and mistic might, as they lower your opponents skill and raise your own.

 

Oh and yes it adds to accuracy :P

It only affect your and your enemy's accuracy (displays as -x% in the prayer stats menu for your opponent). Doesn't do anything to either one's levels.