From whence the Sanguinor comes, none truly know, yet there are two credible possibilities. The first is that the Sanguinor is the last member of the Blood Angels' Sanguinary Guard from the Horus Heresy, although this seems implausible, even with the long life-spans granted to Space Marines by their genetic alterations. The second, and more widely accepted, possibility is that when the Primarch Sanguinius was slain by Horus, his soul split in two: that half which was full of anguish, pain and doubt over his actions during the Horus Heresy became the Flaws that torture the Blood Angels to this day, while that part which was noble and righteous formed into the Sanguinor within the Immaterium. However the Sanguinor came into existence, the Blood Angels of the present day always see his coming as a harbinger of hope and the Emperor's divine will.
Song - [link]