This mod adds a small underground city and a castle to the northeastern Jeral Mountains. The castle includes a small sidequest that unlocks a player safehouse while the city includes a settlement of outcasts, a vampire den, and a dungeon as well as a number of quests and a main storyline uncovering the history of the original inhabitants of the city. While I'm working on a different mod currently, I am still looking for input on this one, particularly concerning quests and the vampire clan. This mod was originally released on planetelderscrolls and uses a variety of other mods from that site, including:

RDweaponspack1 by rdjeke
Akatosh Dragon Model by Saiden Storm
Alivewaters by windsurfer
Nightshade armor by Skeleton K.
Catsuits by SoA
Skeleton’s Helmets Resource by Ancestral Ghost
Lowerclass Cellar by Wormheart
original URL: http://planetelderscrolls.gamespy.com/View.php?view=OblivionMods.Detail&id=5637#Files

Just a few things to note, you can't get out of the underground city the same way you came in, but there's still a way to leave, you just need to ask around. Also, you'll want a fairly high acrobatics ability before entering the dungeon, and the enemies there are all fairly high leveled (no pc level offset, doesn't matter if you are lvl 1 or lvl 50). Many of the enemies, however, are designed to have specific elemental weakness.