Op back
So we have the 19th primarch, An unambitious man whose geneseed lead either to a apathetic degradation of his troops or a fall of chaos through his slothful behavior. He had the skills and ability to be one of the greatest of the Primarchs, but he held no drive or cause. He was a lazy noble raised on a planet with Imperial knights, and was never tested or forced to face any adversity.
He lead his legion for a brief time, out of curiosity, but he soon delegated the role to his loyal Knight commander. This commander took on the mantle loyally and dutifully, and while the primarch rested or was otherwise unconcerned with the affairs of his legion, the Commander lead with honor and glory for the emperor.
The commander held the qualities the emperor had hoped his slothful son would possess, all except the physical qualities he had given his primarchs.
The slothful primarch only ever rose from his blissful stupor in order to engage those who attack his honor. These moments were far and few
One day, the emperor called the commander's audience rather then his child's, knowing full well the primarch would only waste his time. The primarch did not let this stand, and in his foolish pride, challenged the emperor and lost outright. His honor in tatters, he took his loyal legion and rose up against the emperor. The knight commander, loyal and dutiful, sided with his Primarch knowing he would be cast down like his brothers by the emperors might.
And so, the primarch and the 19th legion were wiped away with a heavy heart. Those who surrendered or chose not to side with their provrenitor were mixed with other legions, and those who would not abandon their progenitor were slaughtered to the man.
The legion, the primarch, and the rebellion were burned from the imperial records, never to show of the civil war that only was a prelude to the Horus heresy that would come later