Archaeus

"'Smell that. Slayer blood. Nothing quite like it in the world, is there? ' — Archaeus"Archaeus is an an ancient demon with strong ties to Angel and Spike's past. He is the original demon that was responsible for siring The Master, with the result that all vampires in the Master's bloodlines- including Darla, Angel, Drusilla and Spike- retain some part of him in themselves, with Archaeus in turn able to influence that part of their essence that harkens back to him. He emerged following the restoration of magic and began to hunt in San Francisco, causing Spike to experience dreams of his murders, but Buffy and Willow were able to track Archaeus by astral projecting into Spike's mind and following the link between him and the killer.