Board Thread:Warhammer 40k General Discussion/@comment-7671639-20130406191336/@comment-6134883-20130408185536

It's in the fluff. The Flame Falcons were one of the doomed products of Mechanicus meddling that resulted in the 'Cursed' 21st Founding. Apparently they got off to a good start but less then a century after their founding in the middle of the final battle of their first crusade some of the brothers burst into flames. They were unharmed and the Falcons interpreted the phenomena as a gift from the Emperor. The Inquistion thought different and sicced the Grey Knights on them. I postulate that a decent sized cadre of survivors managed to escape along with sufficient gene-seed to set up elsewhere (ie: Calixis Sector, the Joisan Reach, the dead planet of Tsode) under a new name; The Phoenix Brethren.

The Brethren call their gift the 'Flame of Sanctification' because it leaves them and any good servant of the Emperor fighting alongside them (or nearby as innocent bystanders) unharmed but reduces the Chaos tainted to cinders. They are very fond of flame weapons for some reason....