This is a mod for a SMP server. If you are not playing on the SMP some of the content of this mod will be difficult to find or use for you. Use at your own risk.
This mod features a nightmare mechanic that is set using commands, and so it's primarily meant to be used for storytelling rather than game play.
/nightmare [enable/disable]
/setnightmaredimension [dimension] [x] [y] [z]
currently that's all this mod offers, but eventually it will feature more items, weapons, some interesting mobs, and finally a boss fight.
I might eventually change it so that the mod content is more usable for game play but it will be highly dependent on how successful the main goal of the mod is.
Again, this content is not normally accessible unless you have cheats enabled and is not intended to improve normal game play, use at your own risk.