Prisoner 89 is Zurfina the Sorceress. We find her at the beginning of book 0 imprisoned in Schwarztogrube, the prison for magic users run by the Kingdom of Greater Brechalon. During the course of the story, we see her capture and the events surrounding it in flashbacks. Of course, Zurfina's sole goal in the story is to escape the prison any way she can, and of course the first way she tries is magic. The fact that she manages to pull together some pretty intense magic in a supposedly magic-proof location hints at her true power.
The story here is an example of one of the problems with writing a prequel. Zurfina's imprisonment isn't disclosed in the regular story (books 1-5) until well into book 5, but readers of book 0 know it all before hand. This would be a big clue to one of the major secrets in the series. Fortunately, it isn't revealed until book 5 either.
Zurfina is prisoner 89 because I was writing it while working on my master's degree and I used my student number. I accidental transposed it. I was actually student 98, but I think 89 sounds better.