"The Promise of Eternity" is a book found in Socrethar's Rise in Shadowmoon Valley. Reading it is a part of the 
[40] The Word of Socrethar.
It was written by Socrethar, leader of the Sargerei.
The Promise of Eternity
The Legion's work must continue uninterrupted. There are those on Draenor who wish to disrupt their plans, and chief among them are the unbelievers - draenei who still follow the Light.
Destroy our kin, and keep our masters' work unhindered. Defeat the draenei, and we prove our loyalty to the Legion. Crush Shattrath, and the demons promise us the glory of immortality!