For the record, this is a trigger that has been a long time coming in terms of being pulled.
We've discussed, at length, for IRL years, about the Middle Mountains and its whole End Times wackadillio. You know, with its highest peak holding the Keep of the First Slayer and the 3rd mega Chaos Gate that could overwhelm the whole world being in there and Grimnir being the only thing holding it back.
Now, canonically, all we had before End Times, was that this was a breakaway set of Dwarf Kingdoms. You've got your Karaz Ankor, and then you had the independent Karaz Ghumzul, but they left back home when the Time of Woes happened and also pronounced a huge curse on the mountains and that they were bad and ever since approximately ZERO dwarfs have ever returned there. EVER. No matter what. And they left. And that they sealed and disguised the doors, etc. Also, people have been trying to loot said sealed up dwarf holds ever since, trying to find them. But there are only ever two results from that: 1 they come back with nothing all grubby and annoyed or 2 they don't come back.
And also, its apparently minerally poor around the edges/foothills/lowlands, which is where most all ever human settlement has ever managed, because its drowning in beastmen, greenskins, and huge dangerous Old World forests (but I repeat myself) on top, and human settlement efforts have never ever managed to be large scale enough to result in them doing any big deep delving. Historically, that is.
I mentioned waffling about it, for a long time. Because either you had straight up a 3rd Chaos Gate on the scale of the two Polar Gates here, and Grimnir, and End Times, or you didn't. And if you didn't, I would have to come up with other stuff to make it still a place that had an independent kingdom of the metaphysically stubborn dwarfs go 'EVERYONE LEAVE. FOREVER' and yet somehow retain mass mega amounts of the more 'usual' Mallus wildlife i.e. greenskins/skaven tunnels/beastmen/bandits/etc. + scrabbly human settlements doing some stuff here and there for 2500+ years.
So...yeah.
Good news, everyone! There is NOT a 3rd Chaos Gate of world-rending proportions that has only ever been held back by the tireless endless infinite work of Grimnir of the Ancestor Gods in the Middle Mountains!