The Vampiric Knights were originally the 'Cavaliers Encarmine', a 21st founding chapter, successor to the Blood Angels, 9th legion.
The legion as a whole suffered from the terrible gene-flaw, but it affected no one, more so than the Vampiric Knights. Their home world was destroyed by Inquisitorial purge, the reasons are unknown. Many people believe the Cavaliers to be tainted by chaos, the Cavaliers will say that the Inquisitor was corrupted by chaos. Nevertheless, they did not all die, those who survived escaped via drop-pod, to their moon Lyneth. There they discovered an ancient horror that had slept undisturbed for centuries. No records remain of what they faced down there, but it changed them, triggered the rage, and the thirst in them. Now their sanity is gone and they thirst for blood and slaughter. No longer the Valient Cavaliers Encarmine, now the hated Vampiric Knights. It is rumoured, however, that one of them still retained his feelings of self. ----That individual is named Brother Cyphurious and likely has psychic powers. He wields a mighty plasma gun, 'the Redeemer'. An apt name for he uses it to bring his fallen brothers to justice, those he can no longer make see reason.