I think plenty enough people more familiar with the source material than I have broken the info down but I'm bored so here's my two cents:
Let's start with the possible actions.
[] ACTION: Turn back
The first, and most typical 'Dwarf' action ('Human' too for that matter) would be to turn back. This is entirely reasonable and probably safe. It's also, barring another twist, going to be rather unsatisfying. I will note, however, that if we don't get answers now, it could very possibly be a plot hook for post-Expedition events, and one that the thread/Mathilde would not easily ignore if we're being honest.
We then have various means of attempting to gain information:
[] ACTION: Wait and see
Firstly, we can just linger for a bit and see if anything else happens. I wouldn't count on the Beastmen doing much other than perhaps charging at us (not ideal), but the other Kurgan tribes (the Kul and whoever else it was) might appear and trigger something themselves. Or constant observation w/ Magesight might spot a clue. My impression is that Morghur isn't going to just look up and charge, so I don't think this would be any more dangerous than chilling in the Chaos Wastes would be otherwise. But it's a bit dull and unlikely to really result in anything.
[] ACTION: Scout the outskirts of the crater w/ mobile forces + Mathilde. Look for other, hopefully more benign, entities to make contact with.
-> It's a bit of a compromise pick, but we could just try scouting around outskirts of the crater. My concern would be whether entering the crater would break some sort of figurative/magical tripwire here. Also that I wouldn't expect to find anything other than bones. But maybe there'll be a sign of Dwarvish presence somewhere else, or we can make contact with the other tribes 'challenging' Morghur.
[] ACTION: Mathilde covertly infiltrates the forest and, if possible, Karag Dum by herself
Secondly, Mathilde can attempt to covertly infiltrate Karag Dum or at least the forest surrounding it. This is very much unknown unknown territory here, pitting Mathilde's stealth skills against a big question mark.
[] ACTION: Rangers + Mathilde covertly infiltrate the forest and, if possible, Karag Dum.
Thirdly, Mathilde could move in with the Rangers, including perhaps Snorri. This doesn't really benefit the stealth component, but if we're operating under the possibility that vaguely Order-aligned Dwarfs are still in control of the Hold, Dwarven presence might very well be important in not getting ganked, particularly in comparison to a lone magic-wielding human.
[] ACTION: Move back (either in force or with scouts) to interrogate/question local tribes as to what the fuck is going on.
Fourthly, we can double back and try to interrogate the local tribes. This gets us away from Morghur and can probably shed a little bit of light. But nothing they tell us will be reliable info.
[] ACTION: Ask it and find out.
Finally, we can actually be mature, humble, responsible adults and just walk up to Morghur and fucking ask it
for directions what's going on instead of prevaricating. I mean, Borek seemed to get along with the entity so it can't be that bad, right?
And finally,
[] ACTION: Drive closer and try to hit Morghur with your sword.
Now in terms of trying to deduce what's going on ourselves. I feel like there are two distinct but interconnected questions here: First, what did we just see? Then, what does that mean about the state of Karag Dum?
[] THEORY: It's all an illusion
The mildly disappointing possibility is that that is not, in fact, Morghur standing there. But you'd think that the woman that just solved the enigma of Karak Vlag wouldn't be immediately fooled here. But it's possible. But who could it be?
-> Karak Dum Runemasters
They are a little bit out there. Note that this is orthogonal to whether or not they've been corrupted.
-> Chaos
Duh.
-> Dryad fuckery
This works as a sort of relatively benign explanation for the forest and beastman presence all the way up north, but it's not very clear to me how such an arrangement could have been formed in the first place. Notably, if we're going with dryads as being somewhat, uh, 'nympho' (I'm sorry), then their presence might explain the 'intimate gesture'.
[] THEORY: Morghur is just usual old unconstrained Morghur and there's nothing to see here.
This is the obvious conclusion, but it seems significantly at odds with what we just watched.
[] THEORY: Morghur is bound or tricked by something.
I don't really see the question of whether he's been bound/coerced by force or tricked by an illusion of some sort to be that material, since the possible actors are largely the same. Speaking of whom:
-> Karak Dum Runemasters or Dryad fuckery.
See what I wrote about them above, really.
-> Another Chaos entity.
Could be a powerful Chaos Lord or, like, Malagor or something. Or even the Gods themselves. Note that a rather basic explanation for the 'intimate gesture' might be Slaanesh's involvement here, especially given their known presence in Vlag. An explanation for the clean Waystone flow might be that it was needed to support the Vlag ritual and they haven't adjusted for what just happened yet.
[] THEORY: There's more to Morghur than previously known
Well, this is certainly the most intriguing option, but what could it be?
-> Morghur is tied to Karag Dum by birth/creation somehow.
There's something rather neat about how it was last seen just before the Great War where Dum fell out of contact with the rest of Order, being pursued by people that may well have killed it. Especially when we consider that the Runemasters are thought to have been unconventional and highly knowledgeable of Chaos.
Especially when we know that they knew a major incursion was assembling, had been warning the Karaz Ankor about the coming threat for some time, and that those warnings were being ignored in favour of more immediately pressing concerns. Hence, they may have become more desperate and more inclined to resort to a deeply unconventional solution in order to weather the storm to come...
->Morghur's actually just, like, a top bloke.
Really nice chap.
-> Morghur...
Morghur...
Morg...
Mork (hur hur hur)
oh god oh fuck
