Captain Sahaal was designated by the Night Lords' Primarch Konrad Curze to serve as his successor as leader of the Legion just before his assassination on the world of Tsagualsa following the end of the Heresy in the early 31st Millennium.
However, Sahaal lacked the artefact called the Corona Nox ("Crown of Night") that was required by Night Lords tradition to fully ascend to the position of leader of the Legion, and was forced to begin a ten-thousand-year-long odyssey to recover the artefact and learn the truth about his Legion's fall to Chaos.
When the Horus Heresy began in the early 31st Millennium, Captain Sahaal's loyalties lay entirely with his Primarch, Konrad Curze, also known as the "Night Haunter," rather than the Emperor of Mankind. Like so many of the Chaos Space Marines from this period, Sahaal willingly turned Traitor against the Emperor for the sake of his loyalty to his Primarch and served Curze well throughout the Night Lords' campaigns against the Imperium of Man.
Sahaal believed devoutly in the Warmaster Horus' rationale that the Emperor of Mankind had betrayed Curze and the Night Lords by retreating from the Great Crusade to Terra to seek his own divinity and leaving civilian bureaucrats in control of the Imperium rather than one of the Primarchs who had won the galaxy for him. Before the Night Haunter's death, Sahaal was designated by the Primarch as his heir. However, the Callidus Assassin M'Shen took Sahaal's inheritance of the Corona Nox following her assassination of Curze before the Primarch could declare Sahaal his undisputed successor as the Legion's commander.
Sahaal vowed to gain vengeance for his gene-father's death no matter the cost and pursued the Assassin off-world until the Aeldari ambushed his starship and attempted to take the dead Primarch's crown for themselves. It was during this altercation that Sahaal was trapped in a Warp Storm that lasted for 10,000 standard years in realspace, transporting him from the early 31st to the late 41st Millennium.
Conflict with the Imperium and Night Lords
Thanks to the effects of Zso Sahaal's Sus-an Membrane, he was not slain when the Warp Storm ensnared him. Instead, ten millennia passed in realspace while Sahaal was trapped in the Immaterium and he awoke to discover that his starship had emerged from the Warp and crashed on the Hive World of Equixus. Even worse, the Corona Nox was once again missing from his possession. Sahaal began his quest for the Corona once more and began by killing the Equixus hive gangers who had stolen this precious item from his downed vessel while he still hibernated.
He then discovered a kindred spirit amidst one of the gangs in the lower portions of the hive city of Equixus he had crashed upon, as these gangers reminded him of the people who had inhabited the Night Haunter's (and his own) homeworld of Nostramo.
However, Sahaal was unlucky in that members of the Inquisition's Ordo Hereticus, led by Inquisitor Kaustus, were investigating Equixus at this time for evidence of heresy. Sahaal's quest for the Corona Nox was further complicated by the presence of the psyker Mita Ashyn, a young Interrogator within Inquisitor Kaustus' retinue of Acolytes who could sense his presence and objectives, and so Sahaal led the hiver gang to war throughout the hive city on Equixus.
Sahaal also continued to attempt to regain the Corona and eventually signaled his fellow Legionaries amongst the Night Lords Heretic Astartes now inhabiting a Daemon World within the Eye of Terror to come to Equixus and aid him in his task. He was, however, deeply disappointed when these Night Lords arrived and Sahaal learned that in his absence the Legion had forgot Konrad Curze's teachings of "fear through focus" and instead had devolved after ten thousand Terran years under the constant influence of Chaos into little more than super-powered, fear-craving psychopaths.
The Night Lords were now led by the embodiment of corruption in Sahaal's eyes, a Daemon Prince of Chaos Undivided named Krieg Acerbus, known during the Heresy as the "Axemaster" who had once been Sahaal's greatest rival within the Legion.
Sahaal succeeded in defeating all of his foes and reclaiming the Corona Nox, though at the price of a severely wounded arm. Sahaal eventually returned to the Eye of Terror to press his claim to rule over the Night Lords, but this attempt did not prove successful, as many members of the Night Lords now called Sahaal "The Betrayer" because he had fled from the 1st Company whilst they were still battling the Aeldari so that he could continue to hunt the Corona Nox.
A discussion between Talos Soul Hunter of the 10th Company of the Night Lords and the Warmaster Abaddon later revealed that Sahaal's claim to the leadership of the Night Lords Legion had no more merit than any of the other supposed chosen successors to the Night Haunter who displayed other stolen relics that belonged to their Primarch and claimed to be his heir.
- Artificer Armour
- Mordax Tenebrae - Zso Sahaal uses an ornate Bolter named Mordax Tenebrae, "the Dark's Bite". It had been hand-crafted within the hive city of Nostromo Quintus on the Legion's lightless homeworld of Nostramo and was priceless. It had a finely constructed stock, elaborately decorated chambers and a skull-mouthed barrel. The Bolter had been a gift from his Primarch for Sahaal's loyal, dedicated service.
- Unquis Raptus - The Talonmaster also makes use of archaic Lightning Claws which he named the Unguis Raptus, the "Raptor's Claws" -- and in so doing had coined the name of his command company. These too had been a gift from his master, whose generosity could be as unpredictable and spectacular as his moods. Sahaal had received them as gratefully as he had his Bolter, but had wielded them with far greater relish: finding in them weapons worthy of the precision and purity he craved. Before even the Horus Heresy his Raptors became justly feared, and in the name of first the Emperor, and then his Primarch alone, they had brought swift death from above to their foes. If his master had known where the gauntlets were constructed, or by whom, he had never revealed it. They were as much a part of Sahaal now as were his own limbs or the hate in his soul that drove him forwards.
- Black Crusade: The Tome of Blood (RPG), pg. 30
- Lord of the Night (Novel) by Simon Spurrier
- Soul Hunter (Novel) by Aaron Dembski-Bowden