Accidentally killed one of the major bosses without doing any related side quests - What did I miss?

My first visit to Svartrau was during the night. I was making my way along the rooftops and eventually just found my way into the Cathedral. Made my way across the rafters and happened upon >!Xanthe!< surrounded by a few guards.

I thought "Hey this is interesting. I wonder if I could..." I made a manual save, jumped down, and engaged. I'm playing on the hardest difficulty so I really did not think I stood even the slightest chance. But I won.

I didn't reload my save after that because I just thought it was the coolest thing the game just allowed me to do that, even though I hadn't engaged in anything related to that particular character at all.

But now I'm wondering if I missed anything important or powerful. I'm definitely going to be playing the game again. Probably several more times. Aside from the obvious story beats I would have missed by not doing the quests are there any incredibly powerful items or abilities I definitely missed out on? I don't care about item or ability spoilers.