Lorgar
Lorgar is the Primarch of the Word Bearers legion, the extra religious legion of Kheyos Sphess mahreens. He is credited within WH40K 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 but Lorgar was the first to be actually corrupted, though wasn't noticed as he was very secretive about it.
Pre-Heresy
Back in the day Lorgar loved the Emprah. Like really loved him, in fact he was the first primarch to venerate the Emprah like a God. He loved him so much that he wrote an awesome book about how great he was and built a fuckhueg cathedral to him, along with building fuckheug cathedrals in every last world the Word Bearers conquered for humanity. 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, upon which Lorgar decided love was over and found himself some gods that could really appreciate his stalker like adoration.
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 demon summoning you want to be on. This tends to make chaos's otherwise awful selling pitch seem oddly compelling. He also has a hatred for atheists, so he loathes the weeaboo space communists more than he hates everything else.
Since the heresy Lorgar's notable achievements consist of becoming a demon prince of chaos undivided and then doing sweet fuck all for several millenia. Seriously, at least Magnus pretends to have a plan while he sits in his tower screaming, "JUST AS PLANNED". Lorgar? The lazy fuckwit has just been holed up on Sicarus meditating and traversing the warp. Congratulations Chaos gods, you created the lovecraftian equivalent of that friend who won't get off your couch.