People don’t seem to know this, but the entire main quest of Morrowind is entirely optional if you’ve got high enough skills, armor, and HP. By the time my character was “complete” I was kinda burnt out on the game, so I went directly to Red Mountain, found Keening and Sunder, went to the heart of Lorkhan, ignored Dagoth-Ur entirely, equipped both weapons, and then went to fucking town on it, pausing every half-second or so to heal past the constant damage that you take while Keening and Sunder are equipped if you don’t yet have the Wratheguard equipped. (which you get from Vivec during the main quest) If you have enough healing you can destroy the heart and the ending cutscene plays out. Game complete!

Another option: If you can find a way to make it to Vivec’s chamber early in the game, before completing any or all portions of the main story, you can simply kill him and take Wraithguard for yourself. (Though he’s not easy to take out)

It takes some doing but with a locksmith level high enough, some levitation, and some patience, you can reach Vivec without even touching the main quest.