Esther,

Thanks for the question.  Writing about fantasy, even magic, is not a sin; literature has been filled with it for centuries, especially fairy tales.  That being said, it could  somehow glorify the occult and lead people to that path.  That doesn't sound like what you are doing.

Many Christian writers have employed fantasy, such as C.S. Lewis and J.R. Tolkien, who wrote a fantastic work of fantasy in The Lord of The Rings.  No one would regard it as occult-like.  Other works like Harry Potter have been more controversial in Christian circles, but there is no fundamental consensus on it by all Christians.  Some of the criticism on this last work is that is draws from real spells and occult material in the formulations depicted, which never is a good thing.  Exorcists (I have listened to) haven’t necessarily condemned the work, but they wouldn't recommend it for anyone’s children because of the risks involved.

So, some discernment is needed.  When in doubt, have a trusted friend, who shares the faith, read your work and give you feedback. (or even a good exorcist)

Peace,