It can be found in the Warriors of the Dark Gods army book, downloadable from here:
https://www.the-ninth-age.com/index.php?beta-page/I like it, for the most part. It has some contrivances, but no more than GW. It is, for the most part, like a better thought-out version of the GW Chaos fluff. With dark gods that more closely match the basest impulses of human nature than GW's four gods, even though the latter are supposed to be just that.
What I found particularly amusing was the first part of the story of Ilarion Yanovich, a minor noble assigned command of Totvaros, a border town in Volskaya (a.k.a. Kislev). I don't know if it's intentional on the part of the authors, but I suspect it is: He arrives there and acts pretty much like he would in the Warhammer world, only to discover that things don't work like that here. He is flabbergasted to find out there's a temple to the Dark Gods in the town, orders it boarded up, to the anxiety and consternation of the locals. A bit later, a sizable force of Warriors of the Dark Gods approach the town. Ilarion mobilizes town defences and gets ready to die with his boots on, but to his surprise, the assault does not happen. Instead, a different party of Warriors comes into town every day, one party per Dark God, are reluctantly let in by Ilarion, worship at the temple and trade and interact with the locals, enticing some to join them.
