Favorite "Secret of the 40k Universe"

[Jmaximum]

I also think the Emperor is a fallen Old One, but then so perhaps is Sigmar. But they are not the same person, or are they?
Dosadi

This blows my mind, as perhaps his personality is split not only amongst different bodies, but different timelines as well (although Warhammer fantasy is apparently not set on ancient earth, as 40K would seem to stem from that time period and reality.)
Also, this is kinda of implied/confirmed in the old Ian Watson trilogy Draco, Harlequin, Chaos Child (from 2003). In one of the books the Inquisitor actually has a conversation with the Emperor, or so he thinks, but it is then revealed the Emperor is only using a small fraction of himself to speak with the inquisitor. The Emperor says something along the lines that his attentions are spread everywhere and across time as well.