Toward the end of the quest, you are supposed to speak to Lord Trystan to further the quest after drinking from the memory pool at the back of the hall. But Trystan isn't there. I've seen this mentioned in the forums from a few years ago, but no mention of any fix or resolution. I'd love to finish the quest and it's a long one, so I don't want to abandon and do it again. Those posts had people saying they had done so several times without it working.