I’m looking for a fantasy book I read in the 80’s or 90’s. I believe the author was a woman. The story was about a man that is pulled into a fantasy world by a broken down cripple, to impersonate a guy that disappeared. There’s a stone that’s embedded into his skin, with memories. He has to convince an old wizard, and a beautiful woman. He also has to confront a bad guy. In the end, it’s revealed that the broken down guy is the missing guy, and that he was corrupted and broken by the evil guy.