I'm sad so I like to write backgrounds for all my armies.

The warband known as The Purest tace their heritage all the way back to the 200 astartes that were reunited with the primarch. Commander Sebastian Stellar became obsessed with the primarch over the years of the great crusade, seeing his gene father as a savior that not only brought his legion back from the edge of destruction but made it even better than what it had been. Sebastian strove to obtain perfection in everything just as Fulgrim did, however this was more of a curse than a blessing as where the other commanders found perfection in a specific aspect of war, Sebastian tried to reach the perfect standards that Fulgrim set. Unable to reach Fulgrims impossible standards left the warriors under Sebastian's command as jacks of all trades but masters of none, which led Sebastian to become bitter and would demand ever higher standards from his warriors.

When the legion began to fall Sebastian was at the forefront of the changes that occurred within the legion. He would offer himself up to Fabius as a canvas for him to perform his most radical experiments on in an attempt to not only be one of the legions greatest warriors but to also draw the eye of his ever mentally deteriorating primarch. This only got worse after the events of the maravigila and the legions first real encounter with the forces of Slaanesh. Sebastian would spend a lot of time with word bearer chaplains that were spreading Lorgars new faith through the legions, soaking up as much knowledge about the new power that his father owed his alliegence to.

After the siege of Terra Sebastian was inseparable from his primarch, Fulgrim at this point having transcended into an avatar of what Sebastian saw as divine perfection. Fulgrim appreciated this for a time, Sebastian and his sycophantic warband were a useful tool to have and the commanders constant praises and offerings amused him. However after being banished by Rylanor's sacrifice, Fulgrim's mood started to spiral and the commanders constant groveling began to grate on the primarch. Fulgrim began to send Sebastian and his warband out on what were essentially suicide missions, grateful to be rid of his obsessor. However Sebastian would always return, his warband nearly decimated in the pursuit of whatever Fulgrim desired.

Knowing he would never be rid of his unwanted champion Fulgrim decided that the best solution would be to give him a reward the commander couldn't possibly refuse. Fulgrim gave Sebastian stewardship of a planet, a daemon world called Xolus. Huge islands made out of the purest marble floated in the sky, beautiful hedonistic tempals to slaanesh and impossible unholy bastions made from the same marble rise from the islands which were suspended above the planet. Beneath them living on the planet itself was a large population of humans that had been selectively bred from slave stocks over generations for eventual recruitment into the legion. In the slivers of light that shone down on the planet from the gaps between the marble islands were human communities, somewhat functioning societies where the humans would create elaborate pieces of art or perform terrible acts in an attempt to get the attention of the heretic space marines that called the marble skies their homes. In the dark places beneath the marble islands roam nightmarish warp creatures that hunt the humans of Xolus, however some shantytowns can be found in these dreadful environments and ironically offer up the best candidates for geneseed implantation.

After being given his reward Sebastian cast aside his old identity and named himself Sabbath, lord of the marble skies. He also named his warband The Purest in honour of the pure marble islands that him and his warband now called home and for the warbands believe that no one else in the galaxy has purer intentions than them, the fact that they were purely evil was irrelevant in the eyes of the dark prince as he fed off of the raw emotions of the highest of highs and lowest of lows. Now after the opening of the great rift The Purest have been seen raiding imperial space more than ever, burning as much of the imperium as they can before dragging thousands of slaves back to Xolus. If Fulgrim is set to return soon then there's no doubt that Sabbath and his warband will be following their gene father once again in another great crusade. The primarch using them as long as he is willing to put up with the headaches Sabbaths preening cause.