Critical Strike Ability?

[Darkness-4ever]

Pretty much all I want to know is how to create a critical strike ability similar to Warcraft 3's. It has to have % chance, and a damage multplier, red #'s appearing overhead displaying the crit strike damage aren't necessary, but would be nice, but mainly I just want to know how to have an ability that gives a % chance to deal more damage to enemy targets.

 

[mrzwach]

This tutorial should help you. Just change it so that it deals extra damage instead of a special effect.