The Sanguinor is a mysterious figure that is important to the Blood Angels, who only appears at their most desperate moments.
The Sanguinor first appeared after the end of the Horus Heresy; he is a mysterious figure who bears a striking resemblence to Sanguinius, and after the battle is over, he returns to the Warp as quickly as he appears from it.
The true nature of the Sanguinor is unknown; some say he is a fragment of Sanguinius, others say he is the manifestation of the Blood Angels' collective grief over their lost Primarch. One other possibility is that he is the Sanguinary Guard Aratron, who served as Sanguinius' herald and was lost in the warp. As with Cypher, there's some uncertainty as to whether or not the same individual has kept the title for the past 10,000 years, or is passed to new individuals over time.
The Sanguinor's existence is controversial among orthodox thinkers in the Imperium, especially Puritan Inquisitors. Due to his connection to the warp and unconfirmed identity, they believe that the Sanguinor is a warp creature, sent either to corrupt the Blood Angels, or is an indication of the Blood Angels' own obsessions.
The Sanguinor is discretely in a relationship with Saint Celestine, and the two decided to have some fun by switching roles for a day. So while Celestine rescued a group of Blood Angels from Be'lakor, the Sanguinor rescued some Sisters of Battle. Unfortunately for him, the sisters were none too pleased and began assaulting him immediately, with Celestine having to bail him out.