Celtic Heroes

Someone plz tell me how to get this skill and where at and all I need to do to get it

Anyone able to answer this?

I believe other skills are dropped by OW bosses

Was the boss in the ghost corridor leading to castle where I got mine

Skill books can be drop by every bosses in OW
The one in the corridor is Scion of the lost, maybe it drops this skill more often

Got the spell off OW mob, doesn't do enough to make it valuable for a point ability and definitiely isn't valuable enough to max....goes into the 'don't use' bucket for me until we get into some very long fights where druids are having significant energy issues, which i don't really see happening with the rate that they die during fights.