Lorgar

Lorgar Aurelian, the Urizen. Note that his purity seals have purity seals and his scalp inscribed with holy scripture. Because if you are going to do it, you have to do it right.

Lorgar, also known as Lorgar Aurelian and The Urizen, is the Primarch of the Word Bearers legion, the extra religious legion of Kheyos Sphess Mahreens. He is credited within 40k fluff for being the first Primarch to fall to the ruinous powers. Yes, we know what you're thinking and no, Horus was just the first Primarch to be public about it. Lorgar was the first to be corrupted, though this wasn't noticed as he was very secretive about it.

Pre-Heresy

Like the rest of the Primarchs, Lorgar was scattered through the Warp by the Chaos Gods. Landing on Colchis, a Feral World which worshipped the Chaos Gods, Lorgar was taken in by the priests of the Covenant, the religious order of Colchis, primarily the priest Kor Phaeron. His psychic talent manifested in visions of a messiah coming to Colchis. This pissed off most of the Covenant, so Lorgar and Kor Phaeron organized a purge. By the time they were done, the Emperor of Mankind and Magnus the Red arrived on Colchis and Lorgar hailed him as his lord. After about a month of celebrations, the Emperor appointed Lorgar head of the Imperial Heralds, which Lorgar renamed the Word Bearers.

During the Great Crusade, Lorgar loved the Emprah. Like really loved him; in fact, he was the first Primarch to venerate the Emprah like a god. He loved the Emprah so much that he wrote an awesome book about how great Emps was and built a fuckhueg cathedral to him, along with fuckheug cathedrals in every last world the Word Bearers conquered. Empy caught wind of all this and was not amused, since Lorgar's religiosity threatened to undermine his super sekrit Operation Starve-the-Fuck-Out-of-Chaos. He made Lorgar stop his god worship and told him to go out and conquer the rest of space. By having the Ultramarines destroy a compliant city that epitomized Lorgar's accomplishments, then using his awesome Psyker powers to make Lorgar and the entire XVII Legion to kneel before the Emperor, Roboute Guilliman, and Malcador the Sigilite. Wracked with self-doubt, after punching Guilliman in the face, Lorgar began listening to Kor Phaeron (now First Captain of the Word Bearers) and First Chaplain Erebus about how there might be other Gods that would accept worship. This in turn lead to the Lorgar traveling to a primitive planet called Cadia where he learned the truth about the Emprah's Imperial Truth. The rest is history heresy.

Lorgar has a model now

Before the heresy Lorgar was widely considered the weakest Primarch as both a fighter and warlord, preferring to act more as a diplomat and preacher. His actions during the Heresy proved everyone wrong. When Lorgar first toured the Eye of Terror, he was forced to fight An'ggrath the Unbound (who is to Bloodthirsters what Bloodthirsters are to Gretchin) in order to prove himself worthy of becoming the champion of Chaos Undivided. His plan of attack on Calth resulted in beating the shit out of Ultramarine forces four times his army's size, proving Lorgar was also a damn good warlord. Even the fact that his Calth attack force was finally screwed was part of his plan: the entire point of the Calth war was to purge the Legion from ignorant hotheads who put revenge and hatred over Chaos worshipping. Though knowing what a manipulative bastard he was, his true goal might be turning Guilliman into an (even more) paranoid asshole who would ruin the Imperium from the inside if the Horus Heresy didn't end well for Chaos (Unlikely, he was pretty pissed that Kor Phaeron didn't kill Guilliman when he had the chance.)

He was also terrifyingly powerful psyker, but refused to use his gift for the same reasons. He ceased this emo shit after falling to Chaos, turning into a badass sorcerer able to rival Magnus, or at least Magnus's psychic projection, which is still far beyond any 40k-level monster-psyker capabilities. In fact he was third or forth most powerful psyker in the galaxy: second to only Big E, Magnus and maybe Malcador.

He also bore the greatest physical similarity to the Emperor: under his golden tattoos he looked exactly like a younger version of Big E's true form (yeah, yeah, da Emprah told Corax that he had no true form, but then he also said that daemons were minor xenos pests, soooo...).

GREETINGS PULING CORPSE SLAVE, DO YOU HAVE A FEW MINUTES TO HEAR ABOUT THE GOOD NEWS OF CHAOS UNDIVIDED?

Post Heresy

So now he's the Daemon-Primarch of the Word Bearers, the guys who make the rest of the Chaos Space Marines look like all around swell secular humanist types. They're kind of like Jehovah's Witnesses except instead of knocking on your door and telling to you about Jesus they knock down your door and ask you which end of a daemon summoning you want to be on. This tends to make Chaos's otherwise awful selling pitch seem oddly compelling. He also has a particular hatred for atheists, so he loathes the weeaboo space communists more than he hates everything else.

In a supreme twist on irony, everything the Ecclesiarchy of the Imperium (which is the primary target for the Word Bearers during conquest) derives most if not all of their religious texts from Lorgar. That Bible they administer to citizens daily? The prayers quadrillions of soldiers and civilians utter daily in an effort to believe? That devotion to the Emperor? All fostered and nurtured by Lorgar. All brought about by his and his Legion's sheer devotion. In short, Lorgar is responsible for both the Heresy that marked the end of the Imperium's golden age, and the only thing that would save Humanity in its long evolution to a fully psychic race. That being said, Lorgar is actually so ashamed of the Lectitio Divinitatus (the primary holy book of the Imperium) that bringing it up is a surefire way to have your body and soul obliterated in such horrific ways that no words exist in the myriad tongues of the universe to fully encompass the unholy rape you would receive.

Since the Heresy, Lorgar's notable achievements consist of becoming a Daemon Prince of Chaos Undivided (and, yes, that is technically possible) before doing sweet fuck all for several millenia. Seriously, at least Magnus pretends to be plotting the downfall of Man or something while he sits in his tower screaming "JUST AS PLANNED" at the top of his lungs whenever he pours milk over his daemon cereal, and he actually got off his ass and led the Thousand Sons to wreck the Space Wolves' shit on their own home planet. Lorgar, though? The lazy fuckwit has just been holed up on Sicarus meditating and traversing the warp, secretly "Crying and weeping in guilt for what he has done" quote on quote by our spiritual liege himself, Rowboat Derpyman. Congratulations, Chaos Gods, you have created the Lovecraftian equivalent of that unemployed asshole friend who won't get off your couch and who is secretly emo.

Apparently he was actually working on some new daemon summoning techniques, which he taught Abbadon before the Thirteenth Black Crusade.

The Primarchs of the Space Marine Legions
Loyalist Corvus Corax - Ferrus Manus - Jaghatai Khan Leman Russ - Lion El'Jonson - Roboute Guilliman Rogal Dorn - Sanguinius - Vulkan
Traitor Alpharius/Omegon - Angron - Fulgrim Horus - Konrad Curze/Night Haunter - Lorgar Magnus the Red - Mortarion - Perturabo