Flame Falcons

Flame Falcons
Battle Cry	Unknown
Number	Officially 0, but some renegades may live.
Founding	21st Founding
Successors of	?
Successor Chapters	None
Chapter Master	Dead
Primarch	Unknown
Homeworld	Lethe
Strength	Almost all killed, though several individuals fight on still.
Specialty	Being on fire
Allegiance	Imperium
Colours	Unknown

"No, Falcons. You are the daemons."

– Inquisitor Peter Chimaera

A chapter of the 21st Founding who were, unsurprisingly, some of the unluckiest bastards alive. The 21st founding was a secret founding to eliminate weakness in the geneseed and provide additional benefits. At first, the Flame Falcons seemed perfectly normal and whatever faults had been with their geneseed appeared to be not present. Time would show what a big fat lie that was.

Gene-seed mutation

During a battle, the first company of the Flame Falcons suffered a geneseed mutation that caused them to burst into flames. The flames didn't seem to hurt the Falcons at all, but actually protected them from enemy attacks. Soon the entire chapter was on fire. These flames hurt the enemy, obliterated daemons, and protected the Flame Falcons' allies from harm. Fortunately the flames extinguished themselves after the battle. Everybody thought this was a pretty awesome party trick and blatantly obvious blessing from the Emperor himself that no one but a full-retard asstard could misinterpret.

Inquistion Time!

Some Inquisitor that saw the Falcons make war decided, in a typical grimdark fashion, to spoil everything by declaring the Flame Falcons to be daemons. (The question as to what fucking buffoon would see something that kills demons as evidence of demons is simply answered by either Puritanism and/or Tzeentch, whose mind-boggling convoluted schemes aren't above such kind of manipulations; and a general better safe than sorry! attitude.) Makes you think what went SO FUCKING WRONG with the geneseed experiments that it caused the cells of the Space Marines to spontaneously combust. On the flip side, these guys would love the Fire Lords and Salamanders, and if they didn't get condemned for it, the latter two Chapters probably would've very much liked a refined version of the mutation.

Fast forward, excommunicate traitoris, Grey Knights, everybody dies in a (larger than usual) fire. Which is odd, since not only should their psychic nature have caused the Grey Knights to instantly realize the Inquisitor was wrong, but the flames should have protected the Flame Falcons. Well, the Grey Knights did attack the Space Wolves and have attacked other deeply loyal Chapters for basically no reason. Except none of those times was to kill presumably corrupt Marines but for political reasons and this time is because the Marines in question were accused of being daemons. When they got their asses kicked by fought the Space Wolves and other Chapters, it wasn't due to something like this which, means the Grey Knights would've instantly known their mission was fake. The Grey Knights are perfectly willing to tell the Inquisition to shove it (or kill treacherous Inquisitors stupid enough to manipulate them), a point which we've seen demonstrated increasingly frequently in the wake of Guilliman's return: while still independent and even willing to gainsay him in rare instances, albeit politely, the Grey Knights tend to gravitating to the primarch, and by extension, away from the Inquisition.

Of course, with the Flame Falcons founded during the infamous Cursed Founding, the Inquisitor might have hit the figurative nail on the head. If the Grey Knights uncovered further evidence of genuine or plausible-enough corruption during their assault it would explain why the Chapter was persecuted to such a severe degree - you don't need to openly worship Chaos to be a corrupted puppet, after all, and they're called the *Grey* Knights, not *White* Knights. Even if it wasn't daemonic in nature, an entire Chapter displaying psychic abilities out of the blue might by itself be enough for it to get purged regardless of other considerations, and the overwhelming stagnancy of the Imperium might have been enough for the Flame Falcons to end up targeted for other reasons.

Survivors

At any rate, some Flame Falcons escaped their burning homeworld and are now at large in the Imperium. Although how you will model marines on fire is anyone's guess (maybe convert the also-on-fire Legion of the Damned?). A good example, though, is that at least one Falcon named Gherak signed on as a Deathwatch Blackshield for the Ordo Xenos while being vouched for by Inquisitor Otto Dagover, a Recongregator Radical.

The surviving Flame Falcons, despite being "renegades" still remained loyal to the Imperium and go around saving the day. Chapters who encounter them generally pretend they did not when questioned by the Inquisition. Oddly enough, the Flame Falcons were mentioned as working alongside the Iron Hands, the Death Strike and the Genesis chapter to Exterminatus worlds before Hive Fleet Leviathan could get to them in the 7th edition SM codex. Perhaps the few survivors managed to rebuild thanks to help from other Chapters (like providing equipment and Apothecaries) or maybe the Inquisitor got deemed heretical for the insane judgement of the Chapter and the Chapter was exonerated and rebuilt without GW ever telling anyone.

The Adeptus Astartes probably view them as blessed and a very useful protector of humanity thanks to their burny gimmick.

Although... sorcery is still heresy and does not actually require the sorcerer to be a psyker. It's possible the Inquisition/Grey Knights believed the Chapter was guilty of using sorcery. They might even have been right...Just saying, it's possible but since the Inquisition is against the Chapter the Chapter is probably innocent.

Though over a decade ago, GW suggested that you could try marines on fire and provided rules for them, found here (handily /tg/ updated for 6th edition).