Also, this waiting was valuable in its own way. We've confirmed this is Morghur, and not an illusion.
We have confirmed his aura of instability and terror is quite well shackled, but can still be summoned up.
We have confirmed the purpose of the desert field- Morghur dumped the excess dhar he summoned (to slightly mutate the Kurgan and make them go away, which is interesting) into the bodies after his flock feasted, and then they became clean skeletons shortly after. It seems to be a physical and metaphysical bleaching effect of some sort. The soil becomes sand simply because it's soaked with dhar.
Ultimately, it seems that the Runemaster are using Morghur, whether willing, unwilling or broken, in as responsible a manner as could possibly be expected- they've restricted all of his natural leaking corruption, and he seems to be under very clearly strict parameters to use the minimum amount of dark magic necessary to turn this entire thing into an elaborate ritual instead of a serious siege by becoming a Mutation Gacha.
Essentially, Karag Dum's methods are radical beyond belief, but as a polity they appear to remain uncorrupted. And that's what matters in the end, in this age.