I discovered my humble blog received some attention on another blog, warhammer39999. I'm very grateful for the mention, and it made me realise I have yet to post some background material on my army. So for all you fluff-buffs, here is some history on the chapter!
In the ancient Librarium vaults of Terra, beneath adamantium walls a hundred feet thick, sealed with hexagrammic wards and atomic blast doors, lie those records whose content would drive the common man insane and shake the foundations of the Imperium should they be known to others than the most faithful of men.
Among the heretical works of the Ruinous Powers, arcane tomes of forbidden sorcery and blasphemous xenos texts fortelling the destruction of mankind are stored documents written by the Administratum itself in recent times, documenting the most atrocious crime known to man: treachery among the Emperors' space marines.
The last time the vaults were opened, one new file was added to the mountains of unholy knowledge stored therein, detailing the betrayal and destruction of the Storm Dragons chapter of the Adeptus Astartes.
Unbeknownst to all but themselves and a handful of other chapters, the Storm Dragons were descended from the Ultramarines first founding chapter. During the 9th Founding, captain Adaceus Ajax of the Ultramarines' 7th company, together with fifty chosen companions, left Macragge and took the Ilex system, bordering the Ork empire of Charadon, as their new home. They erected a fortress-monastery, the Tower of Vigilance, on the forest world of Ilex II.
Naming themselves the White Wardens, reflecting their role as new guardians of the systems bordering Charadon, they adopted a white and blue colour scheme, to honour their parent chapter.
For a hundred years the young chapter slowly built its strength, recruiting from the local populace, whose primitive warrior caste offered many potential recruits. With their planet dominated by hostile reptilian lifeforms, the Ilexians were a hardy people, their children primed for battle from age five. Swiftly, the chapter reached fighting strength, and for two hundred years they succesfully repelled Ork invasions spilling from Charadon.
Unknown to the Wardens, however, the Orks of Charadon were only testing the strength of the latest leash thrown around them by the Imperium. When their leader, the Arch-arsonist, deemed his strength great enough to break free from the Wardens' grip, he led a full-scale Waaaghh!! out of his empire, and engulfed the Ilex system with a flotilla of scrap barges and giant asteroid-ships.
The Wardens had barely time to consolidate their full strength at Ilex II and protect their home world. As the Orks encircled and besieged the orbital defences of the planet, the entire chapter prepared themselves for a global war. Meanwhile, support from the Ultramarines and a dozen other chapters were en route to the Ilex system to break the back of the Ork invasion, but it was doubtful they'd arrive in time to prevent the destruction of the White Wardens.
It was then that fate itself intervened.
As the Orks prepared to deploy their ground forces, tendrils of Warp space suddenly ripped through reality, tearing into the borders of the Ilex system. With blinding speed the Warpstorm grew in strength and size, searing through the Ork fleet in a cyclone of nightmarish destruction. In a matter of seconds it had swallowed the greenskin flotilla whole, sucking billions of Orks into the Empyrean. The White Wardens could only watch as their clear skies turned boiling red.
The Astartes fleet sent to relieve the Ilex system of the Ork invasion found itself facing an impenetrable wall of Warp matter, crackling with unreal energies. Even if the Wardens' home world had been spared by its destructive force, there was no way for the Imperium to reach them now.
For a century, the Warp storm lasted, before dissipating as suddenly as it had appeared. As its obscuring tendrils parted and dissolved, they revealed the system miraculously intact, but the Imperium was hesitant to enter the potentially tainted system.
It were the Ultramarines who first sent a scout vessel to Ilex II, after distant-probing sattelites had revealed lifeforms still present on the planet's surface. First contact with the space marines on the surface was cautious. At first, the Ultramarines could not recognise their brothers at all.
Where the White Wardens at first had been proud, knightly Marines in the colours of Guilliman's sons, their armour was now black and green, displaying dubious imagery of dragons and other beasts. On their armour were fitted horns and other barbaric adornments, giving them a feral look akin to the Ilexian tribes. But that was not the worst of it. As the Ultramarine envoys engaged in orbit-to-surface communications with the Storm Dragons, as the Wardens called themselves now, pict-feeds sent by the surviving marines to testify of the horrors they had withstood during their planet's isolation showed them employing xenos beasts in war against the daemonic hordes spilling from the skies.
The once righteous, Codex-adhering chapter had departed from Guilliman's teachings to reduce itself to savage war doctrines, sacrificing their purity on the altar of mere survival.
Presuming the taint of Chaos to have gripped the chapter, the Ultramarines called upon the Ordo Malleus to investigate. However, fate had other plans. As the news of the Wardens' fall from grace made its way to Holy Terra, the Imperium descended into anarchy as the Age of Apostasy set in. In the resulting chaos, the Storm Dragons' supposed heresies escaped from notice, and they would serve the Imperium for another four thousand years.