Board Thread:General Discussion/@comment-10197675-20130806142227/@comment-93.131.213.77-20130809182019

All Dwemer Souls became the brass skin of Numidium. That's one of the more accepted theories. There are, of course, expetions to this, like Yagrum or Radac and the other Dwemer ghosts that can be seen in TES III Morrowind. This is probably because they died before Kagrenac messed around with the heart, and Yagrum was in the outer realms when this happened.