Nemeroth was the Chaos Lord of the Chaos Space Marine Warband, called 'The Chosen of Nemeroth,' and he is the main antagonist of Warhammer 40,000: Space Marine. He is a powerful Chaos Sorcerer. Nemeroth was killed by Captain Titus, right before he was able to ascend to a Daemonhood.
Assault on Graia
Nemeroth leads his Chaos warband in conquering the Imperial Forge World of Graia located in the Segmentum Tempestus. This was a planet devoted solely to the mass production of military ordnance, weapons and vehicles for armies of the Imperium of Man. This planet was also notable for manufacturing Warlord-class Titans - a formidable, massive bipedal-war machine of the Adeptus Mechanicus. Graia is invaded by a large Ork Waaagh! composed of 1 million Orks led by Warboss Grimskull. Instead of risking an escalating and devastating war with destructive weapons of mass destruction or waiting for Imperial Guard reinforcements, the High Lords of Terra charge the Ultramarines Space Marine Chapter with the task of sending a Liberation Fleet to quell the greenskin incursion. Their primary objective is to salvage Manufactorum Ajakis, the primary factory complex where the Titans are manufactured and stored. Captain Titus, commander of the Ultramarines 2nd Company is charged with 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 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 Psyhic Scourge. A warp gate opened and Bloodletters of Khorne emerge, killing the remaining Orks that were facing the Ultramarines, and overwhelm Warboss Grimskull, making him fall.
Inquisitor Drogan, who had been working in support of Titus's efforts, arrived to the scene and approached the Chaos Lord and the daemons. Titus warns Drogan back, but is silenced by a psychic attack and a sarcastic comment from Nemeroth. The Inquisitor calmly walks through the Bloodletters and falls to his knees before Nemeroth. The Sorcerer then appears to extract some sort of "essence" – manifested as purple light – from Drogan, who falls 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 made a speech suggesting that he just absorbed Drogan's memories and knows exactly what has been going on. He then revealed that the Inquisitor was carrying out a plan to use the Scourge to create a Warp gate and launch a Chaos invasion. 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 previously. Nemeroth also revealed that the real Drogan has been dead for a while, 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 power Ork Warboss. The Chaos Lord managed to push the Warboss over a railing again, 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 Invictus and fired at the Orbital Spire in an attempt to close the Warp gate. Titus gave the Power Source to Sidonus and told him to make for a nearby landing pad, then deliver the device to the Fury of Descent, while Titus went off to hunt some traitors. After some fighting to distract the Chaos forces, Titus himself 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 incapacitates Titus again with his psychic powers and lets it slip that the Power Source is "a gift from the Chaos Gods" and that it would allow him to ascend into Deamonhood to become a Daemon Prince. He then commented again on Titus' peculiar Warp resistance, which he thought must be explained by some kind of connection to the Warp. He asked if Titus would be joining them (the Chaos Space Marines) soon and calls him "brother" before disappearing.
Nemeroth`s final appearance is in the end of the game. Titus has climbed the Orbital Spire in search for the Chaos Lord and eventually finds him. Nemeroth tried to lure him to Chaos by offering him untold power, but Titus flatly refused his offer. A titanic battle ensued, with Nemeroth talking behind a protective barrier while Titus fought several waves of Chaos forces. Finally the barrier fell and Nemeroth stepped forward, partially transformed into his new daemonic form, confident in his invulnerability. Undeterred, Titus simply shoulder-charged the Chaos Lord off a ledge, both warriors falling from the great heights of the immensely high Spire. A final battle follows, with Titus chasing Nemeroth in free fall, catching up to him and punching him. Then they become separated again and repeat the sequence. Finally, Titus managed to overpower the half-daemon and crushed Nemeroth's head with his hands. Nemeroth's headless body disintegrates, but his voice is heard repeating, "Will you be joining us soon... Brother?"
Nemeroth wears a Terminator Armor. He is equipped with a pair of Lightning Claws. He is a powerful sorcerer able to summon Daemons.
Warhammer 40,000: Space Marine Website (PC Game)