Shameless plug for my own content. I finally uploaded all 3 versions of my "summoning circle mishap" story. Each successive version is longer and contains more deviant content, but all center around a young woman mage being possessed by a demon she summons.
Be advised that all versions contain sexual content.
Beginner:
http://www.furaffinity.net/view/21294775/
Intermediate:
http://www.furaffinity.net/view/21150294/ (warning: f2m tf)
Expert:
http://www.furaffinity.net/view/21294874/ (warning: futa, f2m tf)