Curate, connect, and discover
Okay so if Donella is basically unable to use or be affected by magic in any way, then what if Ulla was the opposite? I always found it weird how Donella was the one to trap Ulla in the library, cause like. when would she have the time to trap her. she's gathering her troops, so she's a bit busy.
SO, to fix this, I decided that as a side affect of keeping Donella out and away, Ulla begun to rely on magic to continue living. She's technically half-alive though. She doesn't have a complete physical form, so she often appears see-through and its really disturbing.
The way she learns she can possess things is when she's reading a book and she's not able to focus, so she thinks if she possesses it, she'll just absorb all the knowledge. It works, but it makes her feel incredibly sick (not like she can feel sick anymore), and she didn't learn a thing about the book. So when she possesses Varian, she can't access his thoughts or anything.
She kinda destroyed/shut down the portal after throwing Donella out, but she knows that she's practically bound to the library, so if the library was ever unlocked again, she knew that if she left, she'd just disintegrate. Possessing Varian would probably be easier than possessing anyone else, because another of my HCs (that I may have stole from VTT) is that Var has a slight connection to the moonstone, and it's easier to possess magical things if you are magic. The same does not apply with proper ghosts, as seen in s1 with Ruth.
The library would also stop working properly for Ulla, mainly because it didn't like her, but also because she was living off of too much unstable magic, making the whole library unstable. Really putting into perspective how powerful Ulla is.