I glanced around and saw that there have been similar bugs reported, so sorry that this is a repeat. But I didn't see any actual fixes.
I seem to be unable to progress at all. I completed the Dark Castle, got the necromancer gem (the crown reflects the new features), and then nothing. The only quest in my Quest Log is "Arwin Merton's Story". I don't have a quest telling me to 'follow my intuition'. I don't have any NPC's that I can see with a quest marker above their head. I am about to just give up on this save file. Please help.
Since other's have mentioned certain triggers, here are some steps I took to get to where I am.
Arrived at the castle. Immediately went East towards the crypt to bypass the courtyard/wall. Never saw the front door initially. Progressed through, failed talking to Lizzaria who sent me to Aksha. Talked Aksha down, then killed her. Backtracked through the castle to explore/clear it. Proceeded to the master. Killed him. Teleported back to Caer Cyflen. Then stuck.
Here are some screenshots to show context/proof. https://imgur.com/a/tBCcu6O
Does anyone have any idea what's going on? Or is this a dead save file?