For some context, Chaos is seen as something very, very bad in the 40k universe.

The boy who would become Erebus was born to common parents on the world of Colchis in the late 30th Millennium. Even at a young age he felt it was his purpose to manipulate, deceive, and sow discord among his fellows. He felt a thrill testing his own limits with such immoral acts, which as a boy first manifested in his teasing and torturing of Colchis' desert scorpions.

His parents often scolded him for his troublesome behaviour and lack of educational achievements like his peers, comparing him in particular to the boy who was the pride of his village -- a highly religious and studious child named Erebus.

Taking his parents' advice to be more like Erebus perhaps a little too literally, the boy strangled the more accomplished child with a garrote and replicated Erebus' facial tattoos. Taking Erebus;s identity for his own, the boy departed his village and became an aspiring priest of the Covenant of Colchis in the planetary capital city of Vharedesh.

Erebus cared little for his religious studies at first, but gradually began to appreciate their spiritual insights. One day he saw his face reflected into four parts within a cracked mirror, and he realised in that instant that the gods of the Covenant, who were actually the Ruinous Powers, had always been with him and guided his actions.

Erebus became a devout servant of Chaos thereafter, mastering the tenets of the Old Faith of Colchis which in had always simply been yet another of the myriad guises of Chaos worship. When the Covenant of Colchis fell to the control of Lorgar and his followers, Erebus went into hiding for a time before reemerging as an acolyte of the primarch's new direction for the faith.

When the Emperor arrived on Colchis Erebus was already well aware of His threat to the Chaos Gods as the being they named "the Anathema," and was the only mortal on Colchis not overcome by His glory and psychic aura. Instead, Erebus felt exhilaration at seeing the man he knew that he would one day bring low.

As a servant of Lorgar, Erebus was among the young Colchisians who was transformed into a Space Marine of the Imperial Heralds Legion in the first intake of native Colchisians shortly after. By the time Lorgar renamed the XVII Legion the Word Bearers, Erebus had already set into motion the Dark Gods' plans to stop the Emperor from initiating a new golden age for Humanity that would see their influence greatly diminished.

At the end of the Great Crusade, Erebus held the prestigious position of first

chaplain of the Word Bearers Legion. He was a fearsome and intimidating Astartes warrior, with whole sections of the Book of Lorgar tattooed across his shaven scalp to terrify his enemies.

But when Lorgar grew angry with his father after he and the XVII Legion were chastised on the world of Khur with the destruction of the "Perfect City" of Monarchia for trying to spread worship of the Emperor of Mankind as a divinity across the galaxy, Erebus was ready.

He explained to his master -- alongside Lorgar's foster father First Captain Kor Phaeron -- that there were other entities in the universe that were more than willing to accept his worship and the worship of other Humans.

Several decades before the outbreak of the Horus Heresy, Lorgar, Kor Phaeron and Erebus had fully corrupted the Word Bearers to the service of Chaos, even going so far as to erect temples to the Dark Gods on every world they brought into Imperial Compliance. Before long, the Ruinous Powers sought to act through Erebus to set their plan in motion to destroy the Imperium of Man and the greatest champion of Order in the galaxy--the Emperor of Mankind.

To achieve this outcome for his dark masters, just before the Imperial war with the Interex began during the final stages of the Great Crusade, Erebus began to have many secretive dealings amongst the Luna Wolves Legion and with the Warmaster Horus in particular. In fact, it was he who sabotaged the peaceful negotiations and precipitated the war between the Imperium and the Interex, by stealing an archaic Kinebrach Anathame artefact from the Interex's closely-guarded Hall of Weapons. The theft of this blade was not an act of misplaced greed -- it was part of Erebus' carefully-managed plan to corrupt the Warmaster and bring him under the influence of his own secret masters, the Ruinous Powers of Chaos.

By supplying the Anathame to the Traitor Planetary Governor of the planet Davin, who was subsequently corrupted by Nurgle, Erebus hoped that Horus would be wounded by the weapon and put beyond the care of his Legion's Apothecaries. Once this had taken place, the grief-stricken Abaddon and Horus Aximand of the Luna Wolves' Mournival were only too keen to turn over their master to Erebus' care, and Horus was resurrected by the dark Chaos rituals of the Davinite priests within the dark halls of the Serpent Lodge.

During that ceremony, Erebus entered Horus' visions and served as the proxy for the Chaos Gods as they showed Horus visions of the

future intended to inflame the jealousy and arrogant desire for power that had grown within him since the Emperor had left him in charge of the Great Crusade. In the course of this seduction, Horus accepted the offer of the Chaos Gods to rebel against the Emperor and in return they healed him of his wound. From that day afterwards, Horus Lupercal was the pawn of the Ruinous Powers.

After the Warmaster's resurrection on Davin, Erebus continued to whisper his lies into Horus' ear, manipulating the Primarch's arrogance, ego and desire for power and directing him further from the ideals of the Great Crusade. Erebus emboldened his desire to pursue his own ascension to replace the Emperor as the Master of Mankind. By showing the Warmaster the powers of the Warp, and urging him to make pacts with the Chaos Gods, he eventually set Horus on the path to heresy against the Emperor.

Along with Horus, Erebus masterminded the Istvaan III Atrocity that resulted in the massacre of the Traitor Legions' remaining Loyalists on that world, as he was ever mindful of the need to eliminate those elements of the Traitor Legions who still served the Emperor before the war against Terra could begin.

Captain Garviel Loken of the Sons of Horus suspected Erebus was a secret Traitor to the Imperium, which only led the corrupt First Chaplain to add him to the list of doomed Astartes brethren who were to be caught in the Virus Bomb assault on Istvaan III.

Erebus remained with Horus throughout the Horus Heresy, acting as his dark spiritual adviser and co-conspirator. Erebus, however, only ever truly served the Dark Gods of Chaos.

Erebus later proved instrumental in the aftermath of the Battle of Calth when he successfully summoned the Ruinstorm, the great Warp Storm that effectively split the Imperium in two for the duration of the Heresy.

During the Shadow Crusade, Erebus manipulated the powerful Word Bearer Argel Tal through a series of events that led to his death when Erebus viciously stabbed his Athame dagger into the Crimson Lord's back, killing him. This was because the First Chaplain foresaw that the commander of the Possessed Chaos Space Marines known as the Gal Vorbak and the Vakrah Jal would prevent the World Eaters Legion's First Captain Khârn from achieving his ultimate destiny of becoming the Blessed of Khorne. Such a fate would ensure that the Traitors lost the war against the Emperor.

As Kharn and Tal were close comrades, Erebus was nearly killed by the World Eater

once he discovered the First Chaplain's hand in his friend's death. Erebus was viciously beaten by Kharn before being forced to teleport himself from the World Eaters' flagship, the Conqueror.

Erebus again suffered humiliation after the Battle of Signus Prime, which had been another of his plans to corrupt one of the Loyalist Primarchs, in this case Sanguinius of the Blood Angels Legion. Erebus had been warned by Lorgar that this attempt would fail as Sanguinius would never turn against the Emperor, but he had convinced Horus to order the Blood Angels to Signus Prime anyway.

Following Sanguinius' victory over the daemonic forces that had infested the Signus Cluster, in his anger Erebus arrogantly blamed Horus for disrupting his plan to convert the Blood Angels and Sanguinius to the service of Chaos. Angered by the Word Bearer's insolence, Horus flayed Erebus's heavily-tattooed facial flesh from his skull with his own athame.

After the defeat of Horus at the Battle of Terra, Erebus fled into the Eye of Terror with the rest of his Legion. Erebus is currently a Dark Apostle of the Word Bearers Traitor Legion and a member of the Legion's Dark Council, the ruling body of Sicarus, the Daemon World ruled by the Word Bearers within the Eye of Terror.

Erebus is engaged in a near-constant struggle with his ancient rival Kor Phaeron for control of the Word Bearers as the Legion's Daemon Primarch Lorgar remains locked away in deep meditation with the Chaos Gods and has not directly taken a leadership role in guiding his Legion in centuries.

During the 13th Black Crusade of Abaddon the Despoiler in 999.M41, Erebus led an army of Word Bearers across the Cadian Sector, sacrificing millions of human captives to summon a vast army of daemons to assault Imperial forces at the Cadian Gate and across the rest of the sector.