Underdark has had many revisions over the years, and Im never happy with any of them.
The original idea behind Underdark was to create a roguelike set in the Forgotten Realms Underdark, with all the nasties that the FR-U setting implies.
This idea has been moved away from more and more, while it still retains the Underdark, it no longer means Forgotten Realms Underdark, rather it means the deep dungeons of a roguelike.
From the begining (somewhere back in 1998), it was written to use a skill system, rather than the traditional D&D stats of 18, dice type system.
All stats and skills are from a 0-100 rating, 100 being a perfect skill. This makes programming it fairly easy. (I wont say anything about trying to balance it!).
Underdark is written and distributed under the GNU GPL license.
It should be portable across FreeBSD, DOS and Windows. Via the NCurses or PDCurses libraries
The homepage is http://mega-tokyo.com/underdark