This is a project I'm working on as I also do the next chapter of the Lord of the IInd Legion. Basically at some point…earlier this year I walked through some of the canon chapters with "Unknown Gene-Seed" or little written about them to say they belong to the lineage of my homebrewed IInd legion, and I've always loved the idea of the Flame Falcons. Though admittedly they have so very little about them, I pray that they are the ones who are the true Legion of the Damned, because the Fire Hawks Chapter deserve nothing good. Also this was originally going to be a story set shortly after my story Wrath of Faith and Hounds, until I read the absolutely conflicting info on their wiki page. Despite it saying in the wiki the Flame Falcons were purged within a century of their gene seed's mutations manifesting, the battle that listed the Grey Knights purging the chapter on their homeworld of Lethe is stated for sometime in M41. So yeah once again, super non-canon for typical 40k, but for this AU this is what I'm saying happened.
Prelude
Inside the hallowed halls of the Bale Hounds' Fortress Monastery, there is a chamber with banners bearing the marks of their successor chapters, each and every single one is cared for by dedicated savants to maintain a pristine appearance, unblemished by the passage of time. Yet now, towards the end of the 41st Millennium, one banner hangs high above the rest, still tattered and blackened by the charr of war. A war scarred banner of fiery orange, accent with yellow and red threads, with the depiction of a majestic falcon with its wings spread surrounded by golden fire. This is the last surviving chapter standard of Flame Falcons, and it is by the decree of Chapter Master Fynn Mac Ulsterla, known as the Youngest Spear King, that their banner was left unrepaired to never forget the distrust towards the Ordo Malleus. It was once whispered by the druids within the Librarius of the Bale Hounds that when hope is overcasted by the greatest darkness since the accursed Horus Heresy that from the ashes of despair will arise a burning radiant brotherhood. This brotherhood has only ever been described as brothers clad in golden flames and that they would restore the hope that the darkness desperately tries to snuff out. So when the primogenitor chapter learned of the lineage of the Cursed Founding Chapter, it was a belief that many other chapters of the IInd Primarch's blood that the Flame Falcons were this prophesied brotherhood. Shortly after their inception, this chapter of the 21st Founding had earned itself many battle honors, yet now it is only their gene brothers of the IInd legion's heraldry, along with a rare handful of others, that still recall and honor them.
It took a deal brokered between a Rogue Trader and a commander from the Oathkeepers Chapter to unearth obscured records filed away by the Inquisition dated only a century after the Flame Falcon's first deployment. It also became known to the Chapter Masters of IInd legion stock formed in the 2nd Founding that the official declaration of the Flame Falcon's as Excommunicate Traitoris was entered into Imperial Records a full two years after the chapter's destruction and yet was time stamped to have been entered in a century after their inception. Thus have those rare few recovered relics of the Flame Falcons' been distributed to those deemed worthy who number the ranks of the gene sons of Arwyn Clay been a source of the feud that now stands between the Blades of Titan and the Sons of Clay. For it was those who are named for honoring the oaths they swear and the Cu Sith Arcadia that were there to witness the end of an esteemed legacy that spanned four millennia. It was 901.M41, after a grueling campaign in which the Flame Falcon's held out a defensive position against traitor corsairs until only their 1st Company and the shattered remains of three others. They had been relieved of their duty by a flagship of the Oathkeepers supported by a fleet of escorts, followed by more Imperial military forces. The young Chapter Master of the Bale Hounds took the Arcadia Retribution and set out to deliver a relic book written by the Primarch himself. Arg Righ Fynn Mac Ulsterla wished to start his tenure as Chapter Master by being the first in the Chapter's proud history to honor a fellow chapter so far removed from the IInd legion. While the Fire Falcons hoped to return to Lethe, their homeworld, for resupply and recruitment, yet a week later found themselves invaded by forces of the Ordo Malleus. Bold declarations declaring the Falcons as heretics infected by the foul taint of Chaos, it is only by the blessing of the Emperor that we narrowly avoided the small war that threatened to break between the Sons of Arwyn Clay and the Ordo Malleus.
So this will be a three part story, not including this short, very short, prologue and I won't upload the first part until I have probably finished the next chapter of my Lord of the IInd Legion story. Then again I am a creature of impulse so we will see what happens.