I believe the current prevailing theory isBy the way, what's the thread theories on what was going on with Karag Dum? Cor-Dum patted Borek affectionately which is entirely out of character for him, so I think it can't be something as simple as a daemon-binding or an illusion affecting Cor-Dum's perceptions.
There are two directions to look in our search for leads. One is the question of Gods: there is some sort of divine force turning the area around Dum to desert. Some possible suggestions were mentioned here, but it's all very speculative. I think the only safe bet here is that whatever God is involved, it isn't one the Ancestor Gods, but other than that I don't think we have a lot to go on.
The second thing to consider is Cor-Dum. Egrimm makes a good case that if you want to figure out what the dwarves of Karag Dum did to the Shadowgave you first need to figure out what the Shadowgave even is.
On both of those questions we really have no leads. I guess since the Shadowgave has a thing with Athel Loren maybe someone is Bretonnia could have a piece of the puzzle, and if we had to go looking for answers I think that's where I would start, but who would know and why would they share it with us? I think that other than hoping for the answers to somehow fall into our laps all we can really do is keep our eyes peeled for any potential clues.