Total War: Warhammer III

I actually had to look for this one a bit, mainly because I looked southward from my start position. It is actually in the province capital "The Crystal Spires" in the "Plain of Bones" province, west of you. That province is around the middle of the deamon lands, a bit to the west and as far on the top as possible.

Now, if you conquer westward anyway, you will get it on your way there. I didn't and honestly, it isn't necessary, thanks to the Changer of Ways mechanic. I had finished the main campaign and simply transferred it to myself, which kept it at level 5, which it needs to be for the Symposium to be built, so that made it very quick and easy.

Should you not be able to use the Changer of Ways, either because you are at war with he owning faction or it is their capital, you will need to conquer it. How to do that depends on your general situation in the campaign, so there is no quick guide for every one. But it should also not prove too hard, if you play the campaign on easy.