M'Shen was the infamous Callidus Assassin who successfully assassinated Night Haunter, the traitorous Primarch of the Night Lords Traitor Legion. She is known as the only mortal who successfully killed a Primarch.
Following the destruction of his Legion homeworld of Nostramo, Konrad Curze chose the world of Tsagualsa (referred as "The Carrion World"), located on the outer reaches of the Eastern Fringe of the galaxy, as his Legion's new base of operations. The Night Haunter had a grotesque palace built, made entirely from still living bodies. From here, he launched a massive campaign of genocide against the Imperium of Man that made the Night Lords Legion's previous atrocities during the Great Crusade and the Horus Heresy pale in comparison, before or since. M'Shen located Night Haunter on Tsagualsa and managed to infiltrate his palace, stealing into his throne room. Despite Night Haunter's formidable powers of foresight which obviously forewarned him of the assassination attempt, he took no extra precautions to defend himself. It is believed M'Shen was allowed to infiltrate the Primarch's palace, as the Assassin encountered no guards between herself and Night Haunter's throne room. When confronted by the Callidus Assassin's presence, the Primarch allowed himself to be killed to vindicate his decisions to destroy his homeworld of Nostramo and to rebel against the Emperor. Before his death, M'Shen's video-log records Night Haunter's enigmatic last words:
"Your presence does not surprise me, Assassin. I have known of you ever since your craft entered the Eastern Fringes. Why did I not have you killed? Because your mission and the act you are about to commit proves the truth of all I have ever said or done. I merely punished those who had wronged, just as your false Emperor now seeks to punish me. Death is nothing compared to vindication."
The video-log then shows M'Shen leaping forward, although the kill was never confirmed, as the video feed cuts out right before she struck. After the assassination, M'Shen cut off the Night Haunter's head to take to her masters in the Callidus Clade as proof of her kill and many of the Primarch's personal items and trinkets (including the Corona Nox). En route to her ship to escape off-world she was confronted by the Night Lord Apothecary Talos, and declared to him that she would forever after be remembered by historians as the "Godslayer." The Assassin and the Night Lord Astartes briefly fought one another, but M'Shen managed to flee and escape.
M'Shen's death had been foreseen by Night Haunter's precognitive visions. He knew that although his Legion would allow her to escape, Talos Valcoran and then the entirety of the VIII Legion would disobey his final orders and pursue the Assassin in the name of vengeance. As the slain Primarch had foreseen, M'Shen was relentlessly pursued across the galaxy by the Night Lord Talonmaster Zso Sahaal and the former Apothecary of the Night Lords' 10th Company, Talos Valcoran. She eventually met a grisly fate at the hands of the latter; despite initially holding Talos at bay, he blinded her with acidic spit, deafened her with an augmented scream, impaled and disemboweled her with his Chainsword, and finally tore her body to pieces. Despite her death, the video-recorder built into her vambraces somehow managed to reach Terra, containing a recording of the final words and the assassination of the Night Haunter.
- Index Astartes II, ""Bringers of Darkness - The Night Lords Space Marine Legion," pp. 26-27
- Soul Hunter (Novel) by Aaron Dembski-Bowden
- Nemesis (Novel), by James Swallow, pg. 23