[quote=@Nieszka]
Since the Saliszi came from a single tribe of mountain folk, the Arakkai living there now predate the Saliszi empire.

Edit: [@Aristo] Perhaps the original Saliszi were exiled for their practice of human sacrifice by the other tribes of the Godsfang mountains.
[/quote]

That's fine; I guess there could be numerous tribes of people that have called the Godsfangs their home. 
Though it might be a stretch for the Saliszi to have been kicked out - as written in the OP history, they already had Soulsteel by the time they conquered the Rainland, which [i]likely[/i] means they were the dominant power in the Godsfangs beforehand. That's open to interpretation, of course.

I'd imagine the Arakkai have lived under the shadow of the Salished since its founding, and either are too elusive or aren't worth the Empire's time to try and conquer.