[x] THEORY: The specifics are hazy, but this is a known contingency plan that Borek is entirely aware of, but hoped hadn't been enacted. The result breaks all Dawi notions of acceptability, but Karak Dum survives in some capacity and continues to inflict attrition on every local and visiting Chaos force that want to take a swing at them, so it is considered a lesser evil by the pragmatic Karak Dum.
[X] THEORY: The dwarves of Karag Dum are alive and uncorrupted by Chaos, but have resorted to truly drastic measures to survive, probably undertaken by their controversial "Rune Masters", which involves having these beastmen or beastmen-appearing creatures defending the karak for the dwarves. We can assume that this is all in favour of the dwarves by Morghur's easy acceptance and non-destruction of Borek as well as Borek's mutual acceptance of the beastman. The idea of Rune Masters being critically involved is the certainty that some form of magic was used to create this whole situation, including the odd weather, and the Rune Masters' lack of corruption is supported by the lesser level of ambient dark magic in the area, even directly around Morghur himself.
[X] ACTION: We need to verify if the situation here is stable, or if it's something that will come back to haunt the Karaz Ankor and Empire if left ignored.
[X] THEORY: Through either Pact, Binding or some other venue the dwarfs of Karag Dum have installed Morghur as some form of defensive guardian, supported by the fact that Morghur is treating the mountain as a giant herdstone while to my vision it is not. The confounding factor here is that the energy flowing out of Karag Dum is uncorrupted. Morghur himself would almost certainly be using it for his own ends, so it means the occupants of the hold either don't know how to use it, don't care to use it or still consider themselves part of the Karaz Ankor enough that they let the energy flow uninterrupted.
[X] THEORY: This could be a Rune-based Illusion with "Morghur" being a Runemaster. The Dawi of Karag Dum let the forces of Chaos assume that Dum has fallen and is surrounded by Beastmen forces so they don't all unite to attack the Hold together and instead only come in smaller groups to "test" themselves against another chaotic force.
[x] THEORY: That's not Morghur. Morghur constantly changes his surroundings, Morghur does not make reality more stable near himself, Morghur does not display affection, Morghur would never stay here unless he was bound, and if he was bound he'd be struggling constantly to get out. The shuddering of reality can be faked by Runesmiths if they wanted.
[X] ACTION: The forces should move back at least a half-days distance, find a defensible spot, and camp out for a few days to give them a chance to investigate, but no more than two-three days, the overall safety of the expedition still comes first.
[X] THEORY: The Dwarves of Karag Dum did something to burn away the taint of Chaos, much as your Belt of the Unshackled Mountain does, but on a far grander scale. Perhaps it had an effect on the Beastmen here, Cor-Dum included.
[x] THEORY: Whatever the hell is happening, Borek seemed to still care for what Karaz Ankor thought of him and that what we are seeing is the result of his home trying their best to fufill their duties in ways the rest of dwarves probably wouldn't approve. The Corrupter seemed to recognise Borek as almost kin and hasn't made a move against us. I think some very careful investigation is warranted.
[x] THEORY: Morghur was reborn as a dwarf and at least partially protected by the Valayan rites, subsequently those protections were relaxed to allow him to mutate physically while retaining self-control and his dwarven identity.
[X] THEORY: The Dwarfs have control over Cor-Dum and is likely being controlled by Karak Dum's King or Leader, They are doing this to protect Dum and keep Chaos from getting hold of it, there is an ongoing siege to break this defense with a nearby source providing power to the desert to weaken the defenders.
[X] ACTION: Very careful scouting. Both by the knights, dwarves and Mathilde. Starting with inanimate matter and other disposable stuff(like the cows) to determine where the edges-borders of the effect are and just what the hell the effect is without entering it, while the convoy digs in and readies itself to attack, from behind or from the Karag. If that doesn't lead to some clarity, more aggressive scouting may be needed. Either way, Karag Dum is still a part of the Karak Ankor, and there are ways that link them could be used by demons to hurt the whole realm, especially now that Vlag is back and the links reestablished. So while a part of me might prefer as the wise option to turn back, we cannot leave such a threat without establishing its exact nature, if only so I can bring back news of what to do to about it to King Belegar and the High King.
[x] [THEORY]: There are no dwarves to save. Either the "Cor-Dum" we saw is somehow fake, in which case the dwarves hold the place, and need no help holding it; Cor-Dum is real and allied somehow with the dwarves, in which case the dwarves are not worth our trouble as they have allied with chaos; or Cor-Dum is real and conquered Karag Dum, in which case there are no dwarves left to save.
[X] THEORY: There's some sort of strange illusion magic going on, and it's not visible even to extremely good mage sight. Out of all known sources of magic only Dawi Runes are capable of that - so the works of Dawi Runesmiths are still in use.
[X] [Action] Scout the other fronts, ask other expedition members for what they might have happened, check with the other wizards what their expertise tell them about the magical and natural environments.
[X] ACTION: We need information. The Yusak have been in this area for long enough, they must know something of what happened. The wagons will pull back to their territory, and while they are en-route I will attempt to directly ask Morghur (From a distance, using a spell) what has become of Karag Dum. If the worst comes to worst we will already be retreating and I will be fast enough to catch up. We will figure out our further course of action once we know more.
[X] ACTION: Mathilde will infiltrate the hold and look for more answers. In the mean time the convoy should circle the Steam-Wagons and avoid provoking a fight they can't win. If she has not returned within 24 hours they should turn back and attempt to make their way back to civilisation to inform the Karaz Ankor, Empire and Kislev what they found.
[X] THEORY: Morghur has been bound to Karak Dum's waystone, which is acting as a psudo-herdstone. Morghur now sees dwarfs of Karak Dum as allies. This waystone manipulation may also be the cause of the weird geography. Based on Boreks last words, this was a longstanding contingency plan held by the dwarfs of Karak Dum. He is ashamed because it's a desecration of one of the greatest works of the Ancestor Gods.
[x] THEORY: The crater and missing mountains are due to Morghur translocating or teleporting here. Physical signs point to Morghur having been holding the area against nomad raiders for a very long time, and history implies that he's not been seen in the old world since approximately when Karag Dum would have come under attack during the Great War against Chaos. He's been here this whole time. Magical signs and, less reliably, intuition about Borek, point to there being something uncorrupted in that mountain in the middle. Theory continued: Borek expected Morghur, Morghur favors Borek, Borek isn't chaos-aligned(he had the chance to prevent us from acting on Vlag, but he didn't, and was, in fact, happy about the result). Obvious conclusion, Morghur is aligned with Borek in some way apart from alignment to chaos. Less obvious conclusion: Morghur is aligned with Karag Dum because he was a dwarf. That is to say, the first incarnation of Morghur was born a dwarf, far longer ago than merely 300 years ago. And Dwarf souls that never have the rites to send them to the underearth are ejected from the aether, and reincarnated into new bodies to live another life, over and over again. Morghur was originally a secret shame of Karag Dum, his first incarnation was a dwarf that went into the chaos wastes and was changed into what he is now. Possibly someone who followed Grimnir, then had the shame of coming back alive/having his soul corrupted by chaos down to the core so as to cause disaster when reborn, possibly someone else important to Karag Dum's history, important enough to remember rather than burying or forgetting to dull the shame. They have formed a pact with Morghur to defend them and probably isolate them from everything outside, but it is still uncertain whether they've actually fallen into corruption, maybe chaos dwarves now, maybe not.
[X] THEORY: Neither Morghur nor beastmen are what they appear to be - they are instead dwarfs of Karag Dum, who changed bodies through principles they knew but still kept their souls and minds intact. The reason for that was the need for new bodies that could survive fighting in the Wastes. "Morghur" is a member of the Royal Clan or a Runemaster that knew Borek personally, which why he made such an intimate gesture. It is possible that not all dwarfs suffered such fate, if that is the fate they suffered. Borek knew of such plan, but found it shameful and thus fell into despair when he saw it done.