Nemeroth was the Chaos Lord of the Chaos Space Marine warband known as the Chosen of Nemeroth. He was a powerful Chaos Sorcerer who led the Chaotic assault upon the Forge World of Graia in the late 41st Millennium. Nemeroth was killed by Captain Titus, the commander of the Ultramarines 2nd Company just before he was able to ascend to become a Daemon Prince of Chaos Undivided. Nemeroth served as the main antagonist of the video game Warhammer 40,000: Space Marine.
Nemeroth lead his Chaos warband in conquering the Imperial Forge World of Graia located in the Segmentum Tempestus. Graia was devoted solely to the mass production of military ordnance, weapons and vehicles for the armies of the Imperium of Man. This planet was also notable for manufacturing Warlord-class Titans -- a massive and extremely formidable bipedal combat walker that served as potent weapon of the Adeptus Mechanicus' Titan Legions. Graia was invaded by a large Ork WAAAGH! composed of over 1 million Orks led by the Warboss Grimskull in the late 41st Millennium. Instead of risking an escalating and devastating war with weapons of mass destruction that could seriously damage Graia's much needed industrial output or waiting for the eventual Imperial Guard reinforcements, the High Lords of Terra charged the Ultramarines Space Marine Chapter with the task of preventing the Orks from seizing the Titan technology on Graia until a full Imperial Liberation Fleet could be despatched to take back the world. The primary objective for the Space Marines was to salvage the Manufactorum Ajakis, the primary factory complex on Graia where the Titans were manufactured and stored. Captain Titus, the commander of the Ultramarines' 2nd Company, was charged by his Chapter Master Marneus Calgar with carrying out this sacred duty.
But unknown to Captain Titus or the Imperium, the Ork invasion was merely a cover for the Chaos Sorcerer Nemeroth and his warband of Chaos Space Marines to seize experimental Imperial Warp technology present on Graia that he intended to use to become a Daemon Prince and transform Graia into a new Daemon World under his dominion. Accompanied by two members of his command squad, Sergeant Sidonus and Battle-Brother Leandros, Titus joined with the Imperial Guard's 203rd Cadian Shock Troops Regiment and its only surviving officer, 2nd Lieutenant Mira, to eventually kill Grimskull and halt the WAAAGH! But Nemeroth appeared shortly after Titus had killed the Psychic Scourge. A Warp Gate opened and a legion of daemonic Bloodletters of Khorne emerged, killing the remaining Orks that were facing the Ultramarines, and overwhelming Grimskull.
Inquisitor Drogan, who had been working in support of Titus' efforts, arrived at the scene and approached the Chaos Lord and the daemons. Titus warned Drogan back, but was silenced by a psychic attack and a sarcastic comment from Nemeroth. The Inquisitor calmly walked through the Bloodletters and fell to his knees before Nemeroth. The Sorcerer then appeared to extract some sort of "essence" – manifested as purple light – from Drogan, who fell to the ground lifeless. The Inquisitor had been an undead pawn of Nemeroth who used a specially designed weapon powered by the experimental power source Nemeroth wanted to open a portal into the Warp that allowed the Chaos Sorcerer, a horde of Chaos daemons and his Chaos Space Marines to come to Graia.
Nemeroth suggested that he had just absorbed Drogan's memories and knew exactly what had been occurring on Graia. He then revealed that the undead Inquisitor had been carrying out Nemeroth's plan to use the Psychic Scourge to create a Warp Gate and launch a Chaos invasion of the Forge World. The gate, however, was not yet large enough to accommodate an invading Chaos fleet. Titus, still reeling from the psychic attack, attempted to grab the power source during Nemeroth's tirade, but the Chaos Lord took note and redoubled his attack, calling Titus a "clever fool."
But during his psychic assault, Nemeroth took note that Titus somehow seemed resistant to his powers and contact with the Warp. Inquisitor Drogan had also noted this unsuaual quality previously. Nemeroth revealed that the real Drogan has been dead for quite some time, and the "Drogan" the Ultramrines had been assisting was actually a "puppet" animated by sorcerous means. Suddenly Grimskull reappeared and promptly attacked Nemeroth, who grappled with the powerful Ork Warboss. The Chaos Lord managed to push the Warboss over a railing, but Grimskull grabbed him and both of them fell over the edge, leaving Titus with the power source and free to make his escape.
But this was not the last time the Chaos Lord would appear.
Nemeroth next appeared after Titus had used the power source to supercharge the Warlord-class Titan Invictus and fired at Graia's Orbital Spire in an attempt to close the Warp Gate that had been opened upon it. Titus gave the power source to Sidonus and told him to make for a nearby landing pad, then deliver the device to the Ultramarines Strike Cruiser Fury of Descent, while Titus went off to hunt the Chaos Space Marines. After some fighting to distract the Chaos forces, Titus arrived at the same landing pad to find Sidonus defending himself. Before Titus could intervene, Nemeroth appeared behind Sidonus and impaled him with his Lightning Claws, retrieving the power source. Nemeroth then incapacitated Titus again with his psychic powers and admitted that the power source was actually "a gift from the Chaos Gods" and that properly harnessed it would allow him to ascend to become a Daemon Prince. He then again noted Titus' peculiar Warp resistance, which he thought must be explained by some kind of connection to the Empyrean. He mockingly asked if Titus would be joining the Forces of Chaos soon and called him "brother" before disappearing.
Titus confronted the Chaos Lord a final time after he climbed the Orbital Spire. Nemeroth tried to corrupt the Ultramarines officer to the service of Chaos by offering him untold power, but Titus flatly refused such temptations. A titanic battle between the two Astartes ensued, with Nemeroth taking refuge behind a protective barrier of Warp energy whilst Titus fought several waves of Chaos forces. The barrier finally fell and Nemeroth stepped forward, partially transformed into his new daemonic form, confident in his invulnerability. Undeterred, Titus simply charged the Chaos Lord and forced him off the ledge, and both warriors fell from the great heights of the immensely high Spire. A final battle ensued, with Titus chasing Nemeroth in free fall, catching up to him and striking him. Finally, Titus managed to overpower the newborn Daemon Prince and crushed Nemeroth's head with his hands. Nemeroth's headless body disintegrated, but his voice was heard repeating, "Will you be joining us soon... Brother?"
It was a question that left Captain Titus extremely uneasy.
Nemeroth wears a suit of Terminator Armour decorated with the standard heretical iconography of the Ruinous Powers. He is equipped with a pair of Lightning Claws. He is a powerful Chaos Sorcerer who is able to summon many types of daemons from the Warp.
- Warhammer 40,000: Space Marine (Video Game)