"Some memories should have been left forgotten. The faceless remember them all."
The source of the faceless power was their secret library where the Twilight Covenant was kept to maintain the balance of in Ashan.
Due to the fact that their deity is Malassa, they are perceived to be evil by Elrath worshippers, the angels and humans of the Falcon Empire. Hence, during the Elders War, they fought with the angels. The outcome of the war was a draw. Both the faceless and the angels suffered tremendously from their conflict. However, when the angels found their new allies, humans of the Falcon Empire, the angels used the humans against the faceless to banish them once and for all. Losing their members and their source of power, the secret library overtaken by Kiril who brought it into Sheogh, the faceless almost became powerless. However, before they vanquished from Ashan, Tuidhana, founder of the dark elves, made a pact with one of the faceless; hence the dark elves carried on the will of Malassa and the knowledge transcended from the faceless.
In MMH6, it is presumable that shadow elements are the remnants of the faceless since they are seen with the faceless in Inferno campaign and the final campaign of blood reputation.