|Number||Officially 0, but some renegades may live.|
|Specialty||Being on fire|
"No, Falcon. You are the demons."
— Inquisitor 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 mis-interpret until an Inquisitor who saw the Falcons make war decided, in a typical grimdark fashion, to spoil everything by declaring the Flame Falcons to be daemons (although what fucking autist would see something that kills demons as evidence of demons is truly brainfuckingly retarded). 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.
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. 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.
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.
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)