Kinda cheesy but
Name: Loyalist Emperors Children
Symbol: Pre-heresy Emperors Children (clever eh?)
Background: The Legion's history begins with the descent of their Primarch Fulgrim to the resource-poor world of Chemos, where strict rationing and improvisation were necessary to sustain life. It was decided to spare the infant and when Fulgrim grew to manhood he set about reviving his dying world. Old technology was restored, lost areas reclaimed, and the people of Chemos were able to spend time on art and culture. Fulgrim took pride in this, as he harboured a love of perfection in all things.
When the Emperor first arrived on Chemos Fulgrim swore fealty to him instantly, and was placed at the head of his Legion. An accident had destroyed nearly all of the Emperor's Children's gene-seed, leaving only a tiny force of 200 Marines. Fulgrim addressed them upon their first meeting, stating that they would go out into the Galaxy and spread the wisdom of the Emperor. "We are His children," the Book of Primarchs relates he told them, "Let all who look upon us know this. Only by imperfection can we fail him. We will not fail!" The 3rd Legion were named the Emperor's Children from that day, and the Emperor also bestowed on them the right to bear the Imperial Eagle upon their armour, the only Legion allowed to do so. As they were so few in number, the Emperor's Children were placed under the command of the Primarch Horus of the Luna Wolves until they were a large enough formation to crusade independently. Horus and Fulgrim grew close to one another, with eventual dire consequences for the Imperium...
Swollen by new recruits drawn from Chemos and Terra, the Emperor's Children finally mustered the strength to undertake a crusade alone, and Fulgrim proudly led his warriors into the unknown. To many worlds he brought the rule of the Emperor, crushing any resistance in the certain knowledge that any who fought against the Emperor fought against Humanity itself. From the growing ranks of his Legion, Fulgrim selected a few individuals, the bravest, strongest and noblest, to become Lord Commanders, each given authority over the Company commanders. Fulgrim taught the Lord Commanders personally, taking care that they were worthy of the honour of being the representatives of the Emperor. In turn the Lord Commanders passed Fulgrim's words on to the officers under their command, and they to their squads. In this way, through their leaders, each Space Marine of the Emperor's Children Legion followed the Emperor himself. To honour the Emperor, they strove for perfection in all things: battlefield doctrine was obeyed to the letter, tactics and strategy were studied in minute detail and perfected, and the Emperor's decrees were memorised by every Space Marine, adhered to in every way. While the Emperor's Children, like most legions, considered the Emperor a man, not a god, their reverence and adoration for him bordered on the fanatical.
This wish to achieve perfection met its martial zenith during the Cleansing of Laeran, where the Emperor's Children met an alien foe that offensively echoed their ideals. The Laer were judged so formidable by the Adeptus Administratum that it was feared any attempt to subjugate them would take over a decade. The Emperor's Children scoured them from their home-system in a month. This titanic effort was a notable feat of arms perhaps achievable only by Fulgrim's legion. However, it cost them dear; 700 marines perished with over 4,200 being injured.
Commander: Saul tarvitz - As the level of treachery within Horus and Fulgrim became apparent Tarvitz placed himself in the line of fire by willingly going to Isstvan III. There he and all suspected loyalist members would be betrayed by Horus. Horus ordered the bombardment of the planet, but Tarvitz, along with Garviel Loken and Tarik Torgaddon of the Luna Wolves, managed to turn the loyalist survivors into a cohesive fighting force. This "line officer" turned a planned slaughter into a full-scale battle, confounding the great Primarchs themselves. This true child of the Emperor seems to have met his fate on Isstvan III.