Sin of Six Knives
From Warhammer 40k - Lexicanum
The Sin of Six Knives was the name given to one of the eight individuals which made up the mysterious Hand of Abaddon, an organisation within the Black Legion dedicated to reconstituting the Anathame.[1a]
Representing the 6th of the Hand, the Sin of Six Knives was a Daemon or some other Warp-borne creature. Their body constantly flickered in and out of reality and as their name suggests was armed with six blades.[1b]
In the aftermath of the Battle of the Stygius Gilt the Sin of Six Knives was one of the surviving Hand members and accepted Yheng as the new first of their order in the aftermath of the death of Tenebrus.[1c]
Trivia
The creatures association with the number six would strongly suggest it is Slaaneshi affiliated.