This article is about the Sanguinor and theories regarding who he might be. He is a mystery and quite interesting at that if you like 40k background. Personally I think 40k background is one of those things that really draws people into the hobby. There is an army for most everyone that you can identify with on some personal level. I have always loved Blood Angels since they are perfectionists & excel in close combat. As a group they are not an organization you want on your bad side. Blood Angels are willing to make sacrifices to achieve their goals as evidenced by the death of their primarch Sanguinius.
As I alluded to above there are plenty of interesting theories about the identify of Sanguinor. One of the most interesting ones I have come across is that he is actually the C'tan star god, the Deceiver. This theory fits in perfectly with the background behind this star god as he will often assume a false appearance to work his guile, much like Tzneetch... Which is also another interesting correlation but one I won't delve into here. Personally I don't buy into this theory but it is plausible to a certain degree.
There is also another theory that the Sanguinor is Azkellion (sp?), the first captain of the Sanguinary Guard, who took over the role of chapter master immediately following the death of Sanguinius. This would make the first captain somewhat akin to Chaos Space Marines as if he survived this long he would be an ancient and very powerful as evidenced by his rules and background. The Sanguinor was created as an ultimate assassin to kill enemy HQ and he does this very well in terms of game play.
There is also the theory that the Sanguinor is the prodigal archetype of the Blood Angels, a psychic manifestation brought into reality by the absolute devotion of the chapter to their ideals as originally set forth by Sanguinius. The name Sanguinor can be found on the internet if you search for a definition. One I found that has no ties to 40k is an individual that ingests blood & leads a vampiric lifestyle. It's quite fitting & interesting.
Probably GW will never divulge the actual identity of this character. If you like him then play him as he has an awesome ruleset and could easily disappear whenever the next codex is released... Which probably won't occur for another decade.