Looking through the lists of mobs people have posted, I don't see this one. (sorry if i missed it somewhere) Sixth House by Endrek http://planetelderscrolls.gamespy.com/View.php?view=mods.detail&id=2464
During the course of the game, the Sleepers of the Sixth House invite you to travel to Red Mountain and pledge an oath of service to Dagoth Ur. However, anyone who has travelled there knows that Mr. Ur will simply attack you upon your arrival. I was intrigued by the powerful and evil nature of the Sixth House, and the unique structure and artifacts associated with it. It seemed a pity not to be able to choose the darker path and make the world a very different place than you were ordained to as the Nerevarine. Well, this mod is designed to remedy that. You will now have the opportunity to join the Sixth House and perform tasks to rise through its ranks. You'll gain special powers along the way and enjoy some interesting quests and twists. Here are a few of the key features: Join the malevolent Sixth House and serve your Lord Dagoth Ur Start as a lowly Sleeper and rise through the ranks to become Dagoth Ur's dark priest, general of the Sixth House army, and even a Dagoth! Command your own personal Dreamer servants as a high-ranking member of the Sixth House, and eventually control all but the most powerful Sixth House denizens Take on epic quests to thwart Caius Cosades, topple the Imperial fortress of Ebonheart, and even steal Wraithguard from Lord Vivec himself Talk to a Sleeper in Balmora or search the road between Balmora and Caldera to start your hellish journey. IMPORTANT: If you have already progressed through the main quest to the point where you are seeking the corprus cure, you will not be able to join the Sixth House with that character."Without doubt my favorite morrowind mod.. I'd love to see it ported into morroblivion one day (assuming the author allowed).. its such a nice alternative story arc and so fitting with the lore. Enjoy

That sounds like a great mod
That sounds like a great mod - you should port it! :)
Seriously though, there are a ton of great mods for Morrowind, but first there are the more important, but probably less exciting, tasks related to fully porting the complete original game and expansions.
If anyone wants to help out, you can teach yourself the construction set in a weekend, or maybe even a day depending on your level of computer skills - "it's totally not hard" - and once you do that, you can help out in any way that you want, including porting old morrowind mods!
FAQ » How can I help with the project? » http://morroblivion.com/faq#advanced