Acquired from the Main Story at the end of Chapter 2 of the Act VII: Shattered Empire and can also be farmed in the Avalanche Den, Awakened Necropolis and Lair of the Frozen Fang.
It is used as breakthrough material from the Awakened Ivorymoon to the True Ivorymoon weapon (the alternative breakthrough path uses the "Scorpio" weapon from Awakened Necropolis).