The story of the Fire Hawk is one going back almost three centuries now. Curiously enough, the tale did not originate from the hearsay of its concerning Legion - the Crimson Warhawks - but eventually reached their ears instead. Since then, opinions on the veracity of the tale have been varied.
The first sighting, made by a successor Chapter of the Storm Hammers, was reported on a contested moon in the vicinity of Ahzhava. The loyalists were fighting a losing battle against the nightmarish ranks of the Behemoth Guard, when, without warning, a figure clad in Crimson Warhawks' regalia descended upon their foes. Wielding a tattered battle-axe in one hand and a severed helmet (and, presumably, head) in the other, the figure was clad in roiling flames, and strode across the battlefield with apparent disregard for the fact that it had no head. It sudden appearance turned the tide of the battle, and, as quickly as it had appeared, the ghostly warrior vanished into the smoke and flames of the battlefield.
Since then, there have been multiple similar reports. Details tend to vary - sometimes the warrior's armour is described as ornate, other times as simple. Sometimes it looms over any Astartes, while other times it is merely taller than average. Always, it is clad in a veil of shifting fire, and always, it arrives to battle the traitorous forces of the Warmaster.
There are many proposed origins for the being among the Crimson Warhawks, with many disregarding the apparition as a mere fantasy. Others remark that its first appearance is close to the final resting place of Brother-Captain Markus Victaren, who lost his life, and his head, when pursuing an Iron Hearts warbound deep into the Tempestus zone.
Whatever the case, one thing can be said for sure: if the Fire Hawk is real, and its reported sightings are accurate, then it is making a beeline directly for Holy Terra.