Pathfinder Adventure Path Subscriber
Deliberate. You're meant to get it either via discipline substitution (either via trait or martial tradition), archetype or feats.
Aha! Did a global search of the PDF and finally found it. Sheesh...unlike the Book of Nine Swords, Dreamscarred never created a matrix of which class gets what.
So you need to be an Empyreal Guardian (Pg. 158, PoW)