After you slice Dettlaff’s health down, it seems the vampire is just as spry as when the battle began. He vanishes, readying himself to skewer you from behind, when a second figure appears, taking the full force of Dettlaff’s claws, and urging the vampire to return to regenerate elsewhere. Dettlaff departs, and you’re taken aback as the newcomer is instantly recognizable to you; this is Regis, a higher vampire and a trusted friend from your past exploits! Alternatively, if Detlaff beats you, Regis also appears and saves your life.
Shrugging off Dettlaff’s clawing (he heals the wound in moments), you reminisce about how much better he’s looking since the last time you saw him, and can ask how he managed to regenerate if you wish. Also question him about why he is back, and how he knows the killer you are hunting. It seems Dettlaff, as well as being a Higher Vampire, is a somewhat more stubborn (and bestial) fellow than Regis; but Regis is working on him. Answer Regis as you wish regarding his reasons and motivations. It seems Regis has faith that Dettlaff is performing such ignoble deeds for a well-intentioned reason; and spins a quick historic yarn to prove this point.
After asking (if you wish) about what could have angered Dettlaff, you both agree that finding him is the next best course of action. Your chinwagging is interrupted by palace guardsman, and Regis tells you to meet him at his makeshift quarters, at the Mere-Lachaiselongue Cemetery. After fobbing off the guards regarding the trampling of evidence, you are free to move about the warehouse, and the quest concludes.