Board Thread:Skyrim/@comment-11474177-20130808012214/@comment-13446185-20130808071450

Underking (Ysmir Wulfharth, and Zurin Arctus) ascended to Aetherius when he was given back the Mantella during the Warp in the West. When this happened the Underking's soul mixed with Tiber Septim's soul becoming Talos. The reason for this is most likely becuse the Underking and Tiber Septim were both Shezarrines and both were in some way connected to the Mantella.