I think it was a naval invasion. They took that one long before attacking Baktria.

Yes the Horde are the Saka. They were hard pressed by the Phalava and Baktria struck the fatal blow only to be attacked in the rear by the Taksashilians an loosing their core provinces in only a few turns...

What i noticed in most campaigns is that hordes tend to wander around quite aimlessly and slowly declining in size (I guess from battles with AI). But I never saw them successfully resettle. (I remember from vanilla M2TW that the Mongols and Timurids were not that impressive either, not sure if anything can be changed in "horde behaviour")