[KaVir]

There's no easy answer, it would depend on the individual case. It's certainly possible that your MUD might violate copyright and/or trademark law, depending on what you'd copied. It's also possible that the author might actively encourage fan fiction, or at least tolerate it - but equally they might hit you with a C&D and force you to shut your MUD down.

What I would suggest is that you contact them and ask permission. If they say no, better to find out now than later.