After our Legions game with Alex yesterday, Will had asked if there are any xenos factions?
As this is set in the Horus Heresy the official answer is no, but the hive mind that is the internet is not satisfied with that and has generated an exceptionally spiffy unofficial rule book for Orks and Eldar.
https://indd.adobe.com/view/9519cfc6-05d4-4635-8a60-11cdd5e190c7