Looking for a book I started reading a while back, was about like a dark spirit that lived in some Spirit Realm where spirits just wandered until they found cracks to travel through to real worlds. The MC was kind of bonded to mage who kept summoning it, but the mage died and the spirit possessed a girl who was dying in a castle or fort or some such after being taken prisoner for ransom. I think the spirit was called Nix or something, and she was going to cultivate using only dark qi (which was special I guess, everyone else formed cores or whatever from blended elements) and this is kind of where I stopped. Would be really happy if you could me out Cheers!
Well, yeah that's why I'm looking for it, Please don't comment if you have nothing to further the question
Sounds like Nyx - The Shadow Princess over on Royal Road. Be warned, however: if you like the beginning, you will likely be very disappointed by how it develops. Spoiler The first few reviews on the story page cover the issues with the story fairly well. Basically, the story starts out as a brutal, grimdark fantasy. The ancient darkness spirit finally escapes control and takes over the body of a ruined, tortured noble girl. She needs to cultivate from less than nothing in order to get revenge and will make use of whatever she has to, not bounded by human ethics or morality. But the author doesn't let that happen. First, she gets interrupted just before she succeeds in getting enough strength to be semi-competent, forcing her to stage her escape earlier. Then, heavily injured from the early escape, her body's family "rescues" her before she can slip away and start on her revenge plan. Suddenly she's completely useless. She's full of warm fuzzies, is weak as hell, and is basically forced to be a pretty pretty princess for her body's family... who she tells multiple times that she is not the same person anymore, but still insists on treating her as a delusional baby girl and then reacts poorly when it becomes obvious that she really isn't the same person anymore. I gave up part way through "chapter 18" (which is actually chapter fifty-something, the author likes super long multi-part chapters) after the author yet again wrecks an opportunity for good plot with a whiplash plot twist.