Nemeroth was the Chaos Lord of the Chaos Space Marine Warband, called 'The Chosen of Nemeroth,' and he is the main antagonist of the 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.
Nemeroth led the Chaos Forces attacking Graia. He first appears right after Titus had fried the Psyhic Scourge. A warp gate opens and Bloodletters kill the Orks that were facing the Ultramarines, ovewhelming Warboss Grimskull and making him fall. Inquisitor Drogan arrives to the scene and approaches 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. Nemeroth makes a speech suggesting that he just absorbed Drogan's memories and knows exactly what has been going on. He then reveals 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, is not yet large enough for a space fleet. Titus, still suffering from the psychic attack, tries to grab the Power Source during Nemeroth's speech, but the Chaos Lord takes note and redoubles his attack, calling Titus a "clever fool" (whatever that might be).
Nemeroth notes that Titus seems somehow resistant to his powers and contact with the Warp, as Drogan had also noted previously. He also reveals that the real Drogan has been dead for a while, and the "Drogan" the Space Marine had been helping was a "puppet" animated by sorcerous means. Suddenly Grimskull reappears and promptly attacks Nemeroth, who grapples with him. The Chaos Lord manages to push the Warboss over a railing again, but Grimskull grabs him and both fall over the edge, leaving Titus with the Power Source and free to make his escape.
Nemeroth next appears after Titus has used the Power Source to supercharge the Invictus and fire at the Orbital Spire in an attempt to close the Warp gate. Titus gives the Power Source to Sidonus and tells him to make for a nearby landing pad, then deliver the device to the Fury of Descent, while Titus goes off to hunt some traitors. After some fighting to distract the Chaos forces, Titus himself arrives at the same landing pad to find Sidonus defending himself. Before Titus can intervene, Nemeroth appears behind Sidonus and impales him with his Lightning Claws, retrieving the Power Source. Nemeroth 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 will allow him to ascend into a Deamon Prince. He then comments again on Titus' peculiar Warp resistance, which he thinks must be explained by a connection to the Warp. He asks if Titus will 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 tries to lure him to Chaos by offering untold power, but Titus flatly refuses. A battle ensues with Nemeroth talking behind a protective barrier while Titus fights several waves of Chaos forces. Finally the barrier falls and Nemeroth steps forward, partially transformed into daemon form and confident in his invulnerability. Titus, however, simply shoulder-charges him off a ledge and both warriors fall off the immensely high Spire. A final fight follows, with Titus chasing Nemeroth in free fall, catching and punching him, then becoming separated again and repeating the sequence. Finally Titus manages to overpower the half-daemon and crushes 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.