Mike Krause's MUD (Multi-User-Dungeon), Medievia IV, is a derivative of Merc, not CircleMUD (as was stated by Zari), although both of those are derivatives of the original DikuMUD.
Krause claims, essentially, that Medievia IV has undergone too many changes from the Diku stock to be considered a Diku MUD. While it is true that Med has undergone many alterations from, and additions to, the original Merc/Diku code, it is still a derivative of that. It is impossible to play the game without seeing the influence of Merc and DikuMud, and the authors of those games deserve recognition for their work.
Krause is quite clearly in violation of both the Merc and Diku licenses in the way he runs the MUD, by not giving the authors of Diku credit, by not including a Merc help file, and by operating, basically, for-profit (“donations”).
There are probably some other points, but those are the major ones.
The above information was gleaned, in general, from the following two sites. Both are good explanations, written by a former Medievia coder and a former player.