As stated in one of the posts below, which I believe is a quote from Albides:
"Akatosh kills Lorkhan who is him, Hjalti kills Cuhlecain to become him. Hjalti tries to become Ysmir, or Ysmir tries to become Hjalti, both living still. Under request of Hjalti, Zurin Arctus kills Ysmir who kills Zurin Arctus whose heart belongs to Hjalti who becomes Tiber Septim becomes the many-headed Talos. Akatosh kills Lorkhan who is him, Hjalti kills Cuhlecain to become him. Hjalti tries to become Ysmir, or Ysmir tries to become Hjalti both living still. Under request of Hjalti, Zurin Arctus kills Ysmir who kills Zurin Arctus whose heart belongs to Hjalti who becomes Tiber Septim becomes the many-headed Talos. Akatosh kills Lorkhan who is him...
An alternate version of events is that Zurin Arctus is not the other half of the enantiomorph. In this case, it's Ysmir. If Magnus is the observor, then so is Zurin Arctus who is closely associated with him"