Eidolon

After getting his head stitched back on.

"Wait... Didn't you die?"

– Lucius

"I got better."

– Eidolon

Lord Commander Eididdly doo AKA: Lord Commander Primus Eidolon, Lord commander Fuckface, that twat who lost his head, and a lesson on why you don't back talk Daemon Posessed Fulgrim is the most prominent Lord Commander (called a Chapter Master in other legions) in the Emperor's Children Space Marine Legion. A massive, egotistical asshole even before the legion's fall to chaos, which led to him getting a Ferrus Manus makeover courtesy of his daddy. Until Bill glued his head back on and installed a microphone into his mouth, making him the first Noise Marine.

Great Crusade

Eidolon started his career in the Legiones Astartes as a battle-brother of the III legion when it was first founded on Holy Terra. Eidolon was one of the lucky survivors of the Space cancer that blighted the nascent third legion in it's infancy. By the time the third legion had been united with their primarch, Eidolon had been a company captain, and the serious issues he had only started to emerge.

He was really, really, really obsessed with his primarch, and looked to him like a father figure in the creepiest way possible. As such, he was eventually promoted to Lord Commander, and became the second in command of the entire legion.

Battle of Murder

The battle of Murder against the Megarachnids was one of the most brutal campaigns of the Great Crusade (though not as much as the Rangdan Xenocides.) The Blood Angels space marine legion arrived on the world to colonize it, only to get their shit pushed in by the Megarachnid natives. They called for reinforcements, which the Emperor's Children under Eidolon's command answered. Despite being in an advantageous position to attack, Eidolon made so many egotistical mistakes that his force got their shit pushed in too, needing the help of the Luna Wolves to rescue them. After the battle ended, Tarik Torgaddon famously called Eidolon out for being an egotistical asshole who nearly cost the Imperium victory infront of his men. While Eidolon backed down that day, he would never forget the insult.

Horus Heresy

After the cleansing of Laeran, Eidolon was one of the members of the Emperor's Children who had fallen to Chaotic Corruption alongside their primarch. At the time, command of the Legion's many companies was split between himself and his friend/rival, Lord Commander Vespasian, however Fulgrim's analysis of his legion in preparation for the betrayal at Isstvan III concluded that the men under Vespasian's command, amongst them the surprisingly bro-tier Saul Tarvitz were more likely to be loyal to the Emperor as opposed to Warmaster Horus. So Fulgrim murdered Vespasian, and sent the companies under his command to the surface to get virus bombed, however Saul Tarvitz managed to switch places with his battle-buddy, Venerable Rylanor the Dreadnought so he could spy on the battle plan, found out what was going to happen, flew down in a thunderhawk to warn everyone, rallied the defenders, and kept the traitors occupied for a few months so that Nathaniel Garro could make it to Terra. Ultimately though, it was for naught, as every legionaire (except for Garviel Loken, Crysos Morturg and possibly Rylanor) met their ultimate end on the blood-soaked soil of Istvaan III.

After the first bout of Fabulous Bill augmentations and the dropsite massacre, Eidolon died. While en-route to some backwards mechanicum crystal mining world, Eidolon questioned his primarch as to why they were heading to this shitty world of no strategic or material value instead of Terra. Fulgrim who didn’t want to spoil his fabulous plan to build a “City of mirrors” out of said crystals to the rest of the legion, couldn’t come up with a quick enough counter-quip, so he cut the Eidolon's head off and mixed the blood with his wine. Brutal. This change of behavior managed to convince Lucius and Bile that something was wrong with their Primarch, and started taking steps to exorcise the daemon (spoiler: Fulgrim already exercised it with his own strength of will, he was just turning into a coked up sociopath).

Later on, Fulgrim changed his mind and had Fabius glue Eidolon's head back on; the decapitating cut was so perfect that he was able to be revived soon afterwards. However, this did leave him with some side effects, as he was stuck in a constant state of pain and pleasure from that point forward. He was also left with several tics and an awkward gait, which led Fulgrim to order that Eidolon should stay behind him until he could walk more gracefully.

Later, Fulgrim would lead Perturabo and both of their Legions to the Crone World of Iydris to look for an Eldar artifact called the Angel Exterminatus. It turns out it was a ploy, and Fulgrim needed Pert as a sacrifice in a ritual on himself, conducted by Eidolon since he died and came back. Since Fulgrim knew more than he was letting on the whole time, and was much smarter than the fucked up hedonist he was being should be, it's possible Fulgrim planned the whole thing out before he killed Eidolon. What a dick.

Following the Horus Heresy and the Legion Wars, he went on to lead the Phoenix Conclave warband of the Emperor's Children, reestablishing the Legion's old stronghold on Harmony for his own use.