Where can I find the ring that I need to wield Sunder and where is the other weapon that I need to defeat Dagoth Ur.

There is no ring, you need http://www.uesp.net/wiki/Morrowind:Wraithguard and http://www.uesp.net/wiki/Morrowind:Keening

...And you must equip Wraithguard to use either Sunder or Keening. Otherwise they can kill you.

and make sure that Wraithguard is 'activated/attuned'

Ie: just follow the Main Quest