Ok... so there are a number of skills that have a description like "Skill A does x dmg, some other effects. Skill A does y% additional dmg".
For example: Warden bear
Rouse a loyal grizzly to fight at your side. The grizzly's swipe deals Magic Damage.
Once summoned, you can activate Guardian's Wrath for 75 Ultimate. Causes Magic Damage and deals 100% additional damage to targets under 25% Health.
The grizzly's attacks do an additional 12% damage.
Player's Note: while switching weapons bear disappears unless slotted on both ultimate slots
There are other skills with description like that (all sorcs pets, overload, etc.)
Now... y% from what? I'm assuming is not from its own dmg because then they could've just increase the tooltip. This bugs the hell out of me because is so unclear (for me at least).