Since i'm getting bored and have nothing to do, how about we share with each other our own lost primarchs and what happened to them and their legions?
I'll start:
The two lost primarchs are Sevastian Lorax, lord of the 2nd legion, the "War-Bards", and Cretacius Rex, lord of the 11nth legion, the "Extinctors".
Sevastian was the third primarch to be found, after Horus and Leman Russ. He was found on the planet of Arthupia, already conquered by the primarch, who obtained control by gainig the respect and love of the people.
Sevastian was a strong, good-looking man with long, black hair and moustache, and was able to understand almost unnaturally fast a person's true self, allowing his clever mind to always know the right thing to say. Seeing that his father's objectives were pure and his brothers were good men, he happily accepted to join the Imperium.
His legion, renamed the "War-Bards", were known to use less destructive tactics than the other legions, trying to encourage the enemy to surrender and accept the imperial truth peacefully.
Sevastian was a good friend of Magnus the Red, Fulgrim and Vulkan, but was barely able to speak with Konrad Curze, angron and ferrus Manus, who saw him as weak.
However, Sevastian eventually realized that, while his father's objective were noble, his methods were almost tyrannical. Sevastian's methods were, instead, always aimed to his people's health and happiness, almost like the kings of old fairy tales.
Realizing that he couldn't go against his father and he couldn't bring peace and happiness to the whole galaxy, Sevastian slowly fell into a depression, but kept protecting his world and his legion.
Eventually Cretacius Rex, primarch of the 11nth legion, was found on the world of Antrumus, a primitive world populated by cavemen and dinosaur-like creatures. Cretacius had become the king of the entire world and surrounded himself with several wives and hunt-trophies. Cretacius accepted to join the Imperium only after defeating Leman Russ in hand-to-hand combat, being defeated by the Emperor in the same kind of fight and being offered many battles and glory.
Cretacius was a big, muscled man, with an impressive beard. Almost the perfect emblem of manhood, his brutal appearence was ruined only by two small horns on his forehead, a mutation gifted to him by the warp the day that he was separated from the Emperor.
Cretacius was a savage beast in combat, wielding a massive chainsword in one hand and a power-fist with flamer in the other.
Cretacius considered the other primarchs to be weak and Angron himself considered him to be shamefully dishonorable, not different from the spoiled tyrants they fought. When Sevastian first met Cretacius, he was disgusted by his brother's selfishness and egocentrism.
After several years, Sevastian finally decided that he had enough of his father's projects and the unnecessary violence used by some of his brothers. Disobeying his father's orders, Sevastian arrived on the planet where Cretacius was last sent, so that he may face his most evil brother and stop him. Considering his ideals of peace only an unrealizable dream, he renamed his legion the "Utopists" and ordered to paint their armor green, the colour of hope.
Sevastian was contacted by Magnus, who knew his brother's intentions and pleaded for him to stop. When Sevastian refused, Magnus decided, with a heavy heart, to warn the Emperor.
Lorax found the world destroyed, full of corpses of both soldiers and civilians. Rex was celebrating his new victory, along with his legion, by pillaging the world of all of his resources and executing those who previously surrendered.
Soon an all-out war started between the two legions and, after a brutal fight, Cretacius defeated and killed his brother Sevastian, lifting his corpse with his power fist and burning it with his flame thrower. The defeated war-bards retrated but, instead of surrendering, they started using guerrilla warfare to survive the enemy legion and, at the same time, forcing it to remain on the planet.
When the Emperor arrived on the planet, accompanied by Leman Russ, Cretacius called them both weak and foolish, for not punishing the lesser men for their weakness. Rex believed that, since he was strong, he had the right to take what he wanted from everyone that could not stop him, and he intended to show the Emperor that he had become stronger than him. Leman Russ stepped in, challenging Cretacius to a duel to the death, which rex accepted, sure to be able to defeat Russ a second time. However, this time Leman won the fight, decapitating Cretacius as a sign of contempt.
To make sure that no one ever knew that a primarch had killed another primarch, the Emperor ordered that Sevastian and Cretacius were to be forgotten. All records of them were deleted and their legions absorbed by the already numerous ultramarines, so that the high number of marines wold not result suspect.
The Emperor and Malcador, only ones authorized to speak of the two primarchs, started calling them only "the forgotten" (Sevastian) and "the purged" (Cretacius).
While the world of Cretacius was destroyed by exterminatus, the homeworld of Sevastian was spared, being simply deleted by imperial maps, as the Emperor considered Lorax to be just a victim.
The two forgotten legions fought in the Horus Heresy as part of the ultramarines.
After the Horus Heresy and the creation of the Codex Astartes, Guilliman created two chapters composed of the genetic heirs of the lost legions, even naming them the "Utopists" and the "Extinctors", to honour the death of his unremembered brothers, not knowing what crime had been committed by Cretacius.
The Utopists still remembered their primarch and decided that, even if the Imperium was now even worse than how it was during the Great Crusade, it was also the only thing that could save mankind from extinction, and they would have fought for it to keep protecting mankind. The utopists eventually found their homeworld and were welcomed back home as angels of ancient legends.
The Extinctors, instead, soon decided to abandon the Imperium, now lead by weak old bureaucrats, and go into the Eye of Terror to make a deal with the Chaos Gods. After a civil war erupted within the chapter between the followers of Khorne and the servants of Slaanesh, the now warband was united under Chaos Undivide by a powerful chaos lord once member of the Black Legion, who met the extinctors' primarch when he was still a Luna Wolf chapter master.
This my story of the lost primarchs and their legions. Now you tell me your versions of the primarchs, the legions and what happened to them.