“I will enter and live and die here, and deliver the dominion of Shadow Dragons to the Mother, as my atonement.”
Originally pledged to Tiamat, Cor'tanos took his kin and fled the Dragon Wars for what they thought to be a new primal gate... Oblivion. This was a bid for primoridial and nexal god status that would have put Cor'tanos on par with Pha Rann, Demos-Gorgos, and Asmodei. When Time flowed, the eldar saw it as a darkness ahead of them in its flow at the end of this strange thing called 'Time.' They did not understand death for none of the eldar truly died. Also known as the 'purple dragons,' there were many that followed Cor'tanos. In so doing, they did not participate in the Dragon Wars and earned the enmity of both the All Father and Tiamat.
For this, Tiamat considers Cor'tanos and his followers heretics. Their departure meant that Tiamat could not defeat the All Father in the Dragon Wars. Instead of a unified realm of dragons under Tiamat, the dominion of Dragonkind splintered in half, and then most dragons fell to Time and became 'fallen.' Tiamat blames Cor'tanos for this because, had the purple dragons fought with her, she believes they could have defeated the All Father. Cor'tanos' story is told in Malcor's Story and Bomoki's Gate.
To have enough power to pierce the end of Time, Cor'tanos consumed his matriarch, and many of his followers. Those that survived, faced a slow weaking of their will towards the inert state of all spirit and matter past Oblivion: entropy. To the metallics, the shadow dragons are monsters, to be attacked on sight. No dragonkind is more hated than the Shadow clan, by all dragons. This hate is rationalized because of their greed and self-cannibalism to take what everyone thought was a nexal gate. There is more to it than this though. Where most dragons possess corporeal bodies and a breath weapon, purple dragons exist outside of visible light, in the ultra or darkvision spectrum. While they are corporeal and can manifest like this, they travel in two dimensions, from dark to dark. The shadow must be able to hold their form, but small shadows really just slow their travel down. This, coupled to their energy-sapping breath weapon that is barely noticeable, makes shadow dragons eminently powerful and deadly.
To preserve the shadow dragons, Cor'tanos allowed Dar Kell to possess one as a test to see if Tiamat would welcome them back, or at least let them try. Later, Malcor (Kell's son) was also given a shadow dragon. Nearing natural death by Tehran aging and with none of his kin left, Cor'tanos took possession of a young priest of Tiamat named Telleris. As Telleris, the shadow dragon patriarch is now re-learning Tiamat's doctrine and has his sights set on Ascendance to immortality and creating a proper dominion for the birth of a new race, which is preserved in Kell and Malcor's bloodlines.