I last year finished Morrowind by doing the whole main quest and I loved it very muc. Now since I know the stroy I wanted to have a little fun. I created an alchemist. Made very powerfull potions. Got Sunder and Keening. I faced Dagoth Ur. Kılled him once. Went after him. When I use Sunder on Lorkhan I just shrinks and disappears with a spark thus preventing me from using Keening. (Besides Sunder gots broken. I do not know. I can not use It again) So I can not kıll the heart. I can not kill Dagoth Ur. I can not finish the main Quest.
What's Wrong?

Thanks..

I never heard of the heart shrinking before you hit it enough times with both weapons. You didn't use a cheat to set your strength to 9999 or something like that, and killed the heart with one hit?

Sunder is broken? Maybe you used it too much. Go to an armorer or use repair hammers to repair it.

thank you very much. I decreased my strength and both problems are solved..