|Number||Officially 0, but some renegades may live.|
|Strength||Almost all killed, though several individuals fight on still.|
|Specialty||Being on fire|
"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.
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. Cue inquisition. Some Inquisitor 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 fought (got their asses kicked by) 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 willing to tell the Inquisition to shove it (or kill treacherous Inquisitors stupid enough to manipulate them)...perhaps the Grey Knights are immune to corruption because they already are.
Or maybe it was a Grey Knight trick to evacuate the Flame Falcons under the cover of a huge inferno they knew wouldn't hurt their cousins. This theory actually is plausible since the Flame Falcons are still around kicking ass which shouldn't have been possible. Since they also helped with some exterminatus they must also have their ships sooo...yeah, maybe. So, the idiot Inquisitor feels satisfied and vindicated, the Grey Knights keep their useful relationship with the Inquisition (and perhaps an excuse to get that Inquisitor axed for attacking a blatantly blessed chapter and therefore she is a heretic and traitor), and the Flame Falcons get to keep fighting for humanity off the grid. There's also the fact that if the Grey Knights did carry out the extermination order, the rest of the Adeptus Astartes or possibly even the Custodes (given where the Grey Knights' base is located) would have declared the Grey Knights to be heretics and traitors and wiped the assholes out of existence. It isn't like they're necessary since those psyker aspirants would just go to other Chapters instead (increasing those Chapters effectiveness greatly given the thousands of Grey Knights are all psykers).
It's also possible the rest of the Inquisition realize how badly it would go for them if the Inquisition went around offing Chapters like Guard regiments (read: super-human intellect + personal hatred = no more Inquisition, replaced by sneaky Astartes) and quietly rescued the Flame Falcons.
Well, not quite everyone, 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)