It doesn't completely focus on Trixie, though.
It is a story where the caribou invade Equestria and trap Twilight in a cage after taking off her wings. Luna and Celestia are taken to the caribou and encased in Silver and Gold respectively.
Trixie is made into an Alicorn about... a third of the way into the fic I think and she uses blood magic.
I had this faved and accidently erased that bookshelf. How I regret that. So many good stories.
It isn't Fall of Equestria. I know that.

I have been trying to find this as well. Any luck?

Huh. Weeks of looking and I find it right after making my reply.

https://www.fimfiction.net/story/204917/the-trials-of-a-necromantic-equestria