In Vanilla Morrowind, killing Ash Vampires does not weaken Dagoth Ur. This mods attempts to correct that. Now killing Ashvampire weakens him as Bethesda originally intended.

I had been using it for years now. Its time I released it.

http://download.fliggerty.com/download-13-688
http://www.morrowind.nexusmods.com/mods/42239/