Lorgar. also known as Aurelian  or the Urizen . is the Primarch of the Word Bearers. one of the original twenty Space Marine Legions. He was the first primarch to turn to Chaos. and helped to orchestrate the events that led to the ignition of the Horus Heresy. He has since ascended to Daemon Prince status.
The Primarch Lorgar vanished from the gene-labs on Luna while still an infant, along with the other twenty Primarchs. After his spiriting away by the forces of Chaos he was found on a feudal planet named Colchis. The infant Lorgar was discovered by Kor Phaeron who followed the predominant religion on the world, the Covenant. and was raised amongst them. Studying within a temple of the Covenant under Kor Phaeron's tutelage, Lorgar quickly became a devout preacher, his skill in oratory and the power of his charisma winning him many followers. However, as Lorgar grew in standing amongst the people, members of the Covenant began to grow jealous of his popularity [Needs Citation].
Lorgar's youth was plagued by visions of a mighty warrior in gleaming bronze armour coming to Colchis, a cyclopean giant in blue robes standing beside him. At one point, the visions reached such intensity that Lorgar claimed that the prophesied return of Colchis' god was soon to occur. Mustering Kor Phaeron to his cause, he began to preach this news to the people of Colchis, causing disruptions to the rule of the Covenant as people converted to his beliefs. Lorgar's enemies in the Covenant saw this as the opportunity they had been waiting for to remove the threat that Lorgar was to the status quo, declaring him a heretic [Needs Citation].
Those who came forward to arrest Lorgar were killed by his followers. The Covenant split into two factions, and a holy war of immense proportions erupted, eventually forcing the entire population of the world to choose a side. This war lasted six years, ending when Lorgar and his supporters stormed the temple at which the Primarch had trained, killing the monks within [Needs Citation].
The Coming of the Emperor
Less than a year after the victory of Lorgar's people, a landing craft carrying the Emperor and Magnus the Red. along with two Tactical Squads of Thousand Sons Space Marines descended from orbit and landed near the temple. Lorgar was said to immediately recognise these people as the ones in his visions, and swore his fealty to his father and creator [Needs Citation].
Every facet of the Covenant's belief structure was reorganised towards the worship of the Emperor as their saviour, and the people of Colchis united behind their new god. The elaborate celebrations and displays of piety lasted for months, although it was said that the Emperor did not approve of this, wishing to rejoin the Great Crusade as soon as possible. At the conclusion of the celebrations, Lorgar was made commander of the Seventeenth Space Marine Legion. which came to be known as the Word Bearers [Needs Citation].
The Great Crusade
Lorgar led his Legion throughout the Great Crusade. the Word Bearers seeking to eliminate all blasphemy and heresy within the new Imperium. Ancient texts and icons were burned. The construction of vast monuments and cathedrals venerating the Emperor was supervised. The greatest Chaplains of the Word Bearers produced enormous works on the divinity and righteousness of the Emperor, and gave grand speeches and sermons to the masses. The progress of the Word Bearers was slow, but domination of the defeated was complete. At some point during this period, Lorgar penned the Lectitio Divinitatus. postulating the worship of the Emperor as a divine being . This book would later become instrumental in the founding of the Imperial Cult [Needs Citation].
However, the Emperor was not pleased with the lack of progress the Word Bearers were showing, and was even more dismayed at their religious zeal; one of the main goals of the Great Crusade was to free Humanity from the ignorance of religion. And so the Emperor personally reprimanded Lorgar, informing him that the mission of the Space Marines was for battle, not faith. Thus the Emperor ordered the Ultramarines to destroy Lorgar's world-building project on Monarchia. Worse still, the Emperor accompanied by Roboute Guilliman forced Lorgar and his entire legion to kneel before him, humiliating the primarch. Lorgar was said to mourn the Emperor's command for a month, speaking to nobody save Erebus and Kor Phaeron, and wearing only hairshirt robes. The Emperor was about to reprimand the Legion again for their lack of action when news reached him that the Word Bearers had gone back on the offensive, and this time, worlds fell before them in rapid order [3a].
Lorgar and Angron during the Heresy.
It was this event that turned the Word Bearers to Chaos. Whilst Lorgar brooded over the Emperor's reproach, Kor Phaeron and Erebus, his top lieutenants, whispered to Lorgar of the great Chaos gods: beings that welcomed, even demanded zealous worship and devotion. Lorgar was slowly poisoned against the Emperor by Kor Phaeron and Erebus, who were appointed Master of the Faith and First Chaplain respectively, and were tasked with converting
the entire legion to Chaos. After a failed attempt to get answers on the nature of the universe from his close friend and brother Magnus. Lorgar decided to go on a pilgrimage to discover the truth for himself . Eventually Lorgar's psychic abilities lured him to newly-discovered Cadia. insisting he heard a voice calling for him. There, he encountered Ingethel the Ascended who showed him the "Primordial Truth" of the universe. Ingethel convinced Lorgar that humanity had to accept and welcome the Gods of Chaos, and failing to do so would have their race share the same fate as the Eldar. Finally, after being shown a horrifying vision of the future (hinted to be the Emperor on the Golden Throne ) [3b]. Lorgar was finally converted by the Ruinous Powers. [3c] The Word Bearers came to venerate the gods of Chaos, but instead of throwing their support to one god, they worshiped Chaos Undivided. a pantheon composing the four Chaos gods [Needs Citation]. Later, Lorgar journeyed with Ingethel the Ascended deeper into the Eye of Terror where he defeated an Eldar Avatar upon a shattered Crone World and went on to best the mighty Bloodthirster An'ggrath the Unbound in battle, a sign he was chosen by the Dark Gods. After the fight, Lorgar met Kairos Fateweaver who told him that Lorgar now had a choice: he could achieve either personal vengeance for his shame by slaying Roboute Guilliman in the future (a hint at the Battle of Calth. still decades away) but thus doom Horus in his rebellion or put personal satisfaction aside and taste the shame and bitterness forever but work for the greater goal of illuminating humanity with the Primordial Truth. Fateweaver made it clear how vital this decision would be, as this was a rare occasion where both his heads spoke the truth. Contemplating his course of action the last stage of this pilgrimage, Lorgar demanded to see a vision of the future if the forces of Chaos lost the upcoming war with the Imperium. What Lorgar saw changed him forever. 
Now an agent of Chaos, Lorgar dispatched his trusted subordinate Erebus to Davin. There, the First Chaplain orchestrated a series of events in cooperation with Davin Chaos Cult priests that saw Horus corrupted by the Ruinous Powers.  The Legion kept their new devotion secret, until Warmaster Horus declared his own faith in Chaos, and began the galactic civil war known as the Horus Heresy. The Word Bearers quickly joined the rebellion, and many of the worlds they had conquered since their conversion turned as well, having been corrupted by the Word Bearers during their conquest [Needs Citation]. During the Drop Site Massacre. Lorgar and his Word Bearers revealed their true colors along with the Night Lords. Alpha Legion. and Iron Warriors. Lorgar engaged Corax in combat, with the Raven Guard Primarch having the upper hand until the intervention of Curze. [3b]
The majority of the Legion was ordered by Horus to see to the entanglement and possible destruction of the Ultramarines Legion, so that their vast forces could not be brought to bear against Horus' march towards Holy Terra. This was a task the Word Bearers took up with joy, for as the Emperor had chastised the Word Bearers for their faith, the Ultramarines had become his favoured Legion. The assault on Ultramar was led by Kor Phaeron. who swore to utterly destroy the Ultramarines. The Word Bearers ambushed the Ultramarines at Calth. an attack which eventually turned against them. When help from the Ultramarines' homeworld Macragge arrived the Word Bearers were routed from the system [Needs Citation].
During the Heresy Lorgar would team up in the Shadow Crusade with Angron and his World Eaters to strike at Ultramar in an attempt to rectify the traitor failure at the Battle of Calth. Lorgar, recognizing Angron's instability and hidden sorrow over his forced departure from Nuceria, recommended that Angron return to his adopted homeworld to find a sense of closure. Lorgar also had the intention to summon a great Warp storm known as the Ruinstorm which would cut off the rest of Ultramar from the Imperium. When the two Primarchs arrived on Nuceria however they discovered that the slave-lords of Nuceria had massacred Angron's comrades and told the population that he had fled during their last stand. Infuriated, Angron ordered that the World Eaters and Word Bearers purge Nuceria of its population and Lorgar willingly took part in the slaughter.  Eventually the Ultramarines and Guilliman himself arrived, pursuing the traitors from Calth, and the Lord of Macragge battled first Lorgar then Angron. However this had all gone according to Lorgar's plan, who used the immense rage Angron was emitting to begin a Khornate ritual which transformed the World Eaters Primarch into a monstrous avatar of Chaos, allowing him to overcome the slowly debilitating effects of the Butcher's Nails. 
As the Word Bearers battled across Ultima Segmentum. the rest of the Word Bearers were led by Lorgar to Terra. where Horus and his forces were repulsed and defeated after a fifty-five day siege. The Legion took refuge within the Eye of Terror and the Maelstrom. vast wounds in space where the Immaterium leaked into reality [Needs Citation].Source: wh40k.lexicanum.com