Board Thread:Warhammer 40k General Discussion/@comment-25618639-20141119150740/@comment-86.190.171.142-20141231200628

It is also spoken about in the Calth stuff around Oll Perrson (spelling?) etc

The actual mechanism isn't known how or why they come back, it is just stated they do and can return after entire body destruction.

I don't know why the Emperor wouldn't just allow himself to die to re-start in a more...operationally effective body. Perhaps, as the Emperor would have to die during the battle of Terra, albeit towards the end (as it was) the knew that would crumble the Astronomican which was still serving as a beacon for the Dark Angles et al who were in the warp, coming to the hard pressed aid of the Imperials. Losing the warp travel would isolate the Imperialist across the galaxy, allowing the traitors a massive tactical advantage - perhaps too much risk at that time. The loss might even have caused the loss of much of the Imperial assets in the warp, a loss too far given that all resources would be needed to keep the empire together.

Who knows.