The Burning Legion was originally known as the White Scars strike force Attila. They were dispatched to investigate strange happenings on the planet of Metzer VI. There they discovered a series of psychic anomalies that they dubbed as Chaos Vortexes, which were one-way spacial gates to the Warp. This was why Metzer VI was continuously bombarded by demon attacks and had an unusually high number of betrayers.
There was much debate between the leaders of strike force Attila on what to do in this situation. Some thought that the vortexes posed too much of a threat and that the entire planet should be cleansed and destroyed. However, the leader of strike force Attila Commander Gabriel felt that Metzer VI could be a valuable asset because the side effect of these vortexes seemed to be that many of the inhabitants of this planet developed psychic powers. Gabriel felt that Metzer could be developed into not only a recruiting area for future Librarians but also a steady supply psychics that can be "fed" to the Emperor.
Fast forward a few weeks to the Battle of Stygian Reef, one of the darkest days of the Burning Legion's history. One of the outposts of strike force Attila was suddenly attacked by a sudden explosion of demons and Chaos Marines from one of the vortexes. Outmanned nearly ten to one, the marines that were at that base held on as long as they could but was eventually destroyed. The handful of survivers that lived through that tragic day claimed that if it not for Commander Gabriel things would have been a lot worse. Despite the fact that Gabriel was reduced practically to an unrecognizable mass from demonic fires and gunfire he continued to fight on with sheer will alone, but in the end he was also killed by the horde.
After the Battle of Stygian Reef the leaders of battle force Attila made a blood oath that they will carry on with Commander Gabriel's wishes and secure Metzer VI for the Imperium. After many years and hundreds of battles Metzer VI was secured, though the existence of the vortexes demand that Metzer VI be always watched with eternal vigilance. As a reward the current commander of strike force Attila Commander Sebastian was rewarded with the gift of his own chapter, which he dubbed as the Burning Legion in honor of Commander Gabriel.
The fires that mark the power armors of the Burning Legion symbolize not only the courage and sacrifice of Commander Gabriel but also the wrath that the Legion will rain down upon any enemy of the Emperor. To this day the flames that were used to creamte Commander Gabriel's body is considered a sacred object, and is housed in one of the most fortified bases of the Burning Legion on Metzer VI. Whenever a marine shows exemplary actions in battle he is rewarded with some of the flames of Gabriel, which is one of the highest honors that can be bestowed upon a soldier of the Burning Legion. (Which is the reason why all of the banner poles of my commanders and sargeants have torch heads on them)
That is my fluff in a nutshell. Ideally what I would like to do once I have my army painted to have a combination of fast attacks and heavies. The fast attacks such as bikes and assault troops is an indication of their White Scars roots while the heavies are symbolic of the fact that they are often outnumbered and have to sit back and thin down the enemies. The fast attacks will ride forward to hold off the enemy while the heavies will provide them with cover fire. My commander will also be a Librarian to indicate the side effects of the vortexes.
One look at this force and you will know that the Emperor's finest have arrived on the field of battle. Having already won a Best Painted Army for the Rogue Tourny, this force has carved its name in to the history books of the Imperium and Sector Gamus Empirus.
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