Damon Prytanis is a Human Perpetual, though this is not his true name as Damon has long forgotten it. Unlike fellow Human cabal agent John Grammaticus, Damon held humanity in little regard and is shown to be loyal to the Cabal. He even used the Eldar racist expression for humans, Mon-Keigh.
Having been alive for untold lifetimes, shortly after the Battle of Iwo Jima he was recruited by the Cabal and worked as an assassin throughout history. Prytanis was later responsible for rescuing fellow Perpetual Cyrene Valantion from Word Bearers custody and bringing her into the Cabal fold. It took him to die and regenerate several times before Cyrene believed his story. Later Damon accompanied Grammaticus to Macragge on his mission to kill Vulkan with the Fulgurite. During the mission to kill Vulkan, Damon was badly mauled by Konrad Curze, though he used a flask holding a captured Daemon to banish the Night Lords Primarch into the Warp.
Following the events on Macragge, Prytanis was charged with Slau Dha with looking after Grammaticus in an cabal outpost. However this was infiltrated by Eldrad Ulthran and his assassin Barthusa Narek. The Ulthwé Farseer was determined to eliminate the Cabal due to fears that their agenda would lead the Galaxy to disaster. Prytanis was the latest target of their assassination, but the human put up a vicious defense and summoned a team of mercenary guards. Eventually Eldrad and Narek were able to overwhelm Prytanis, and his neck was broken by the Word Bearers. As he was regenerating, Narek shot him with a shard of Fulgurite, permanently killing the immortal assassin.
Prytanis notes that in the past he killed the "Good Man" in Memphis. This is likely referring to Martin Luther King Jr.. Other assassinations include: the "Brother" in The city of Angels (possibly Robert F. Kennedy, the brother of President John F. Kennedy, who was shot dead in the Ambassador Hotel in Los Angeles [Spanish for the City of Angels] in 1968) more than a thousand years later the facts in Memphis, "Holiard" in the "Glass temple of Manunkind" in M19, "Maser Hassan" in the "Spire Terrace" in M22, and Narthan Dume in M30.