It was simple, really -- take a dragon egg, any dragon egg would do -- and then siphon the power of a netherwing dragon, a creature already part dragon, part Twisting Nether energy, and infuse that power into the egg. The resultant dragon was a bizarre mix of regular whelp, black whelp, and nether energy. The red dragon Korialstrasz, along with the blue dragon Kalecgos and a draenei priestess named Iridi encountered the first two of this flight in a battle over the Wetlands. Iridi, dubbing the dragons 'twilight' dragons due to their shimmering amethyst hue, noted that the dragons didn't seem to be affected by powerful attacks, rather they fed on the energy that was fed to them and grew larger with every spell thrown at them, like vampires.
Right -- Iridi, Korialstrasz and Kalecgos soon discovered that the two Twilight dragons were tremendously unstable, and Iridi promptly fed them enough power from the Naaru-given staff she carried that they exploded. So much for the Twilight dragonflight, eh? Not quite. The two twilight dragons that they fought were only the first two that Sintharia had raised. Far from perfect, she'd let them go after they'd escaped her clutches as a test to see whether or not they were worthy. Obviously, they weren't -- which wasn't of much concern to Sintharia, who had already hatched a new whelp and dubbed it Dargonax ("devourer" in draconic).
Dargonax grew quickly, demanding 'food' at a breakneck pace -- but the 'food' the dragon ate wasn't really the conventional kind of food, but rather magical energy, much like his deceased brothers. Sintharia fed Dargonax with more of Zzeraku's essence. In other words, the kid was eating bits of the creature that had made him what he was in the first place.
While only a week or two old, Dargonax was huge -- nearly rivaling the Aspects themselves in size, and incredibly clever and perceptive. Feeding Sintharia what she wanted to hear in a draconic version of 'baby-talk', Dargonax was no mere baby -- he knew that his two 'older' brothers had died, and that they were merely weaker versions of himself, intent on nothing more than feeding their hunger. While Sintharia cooed and fawned over him, he planned to turn on her -- a very black dragon thing to do. But Dargonax was far more cunning than a simple black dragon, and his powers eclipsed anything the black dragonflight had to offer.
Zendarin had his own plans -- instead of meekly following along with Sintharia's wishes at the promise of infinite amounts of magic to siphon, he wanted Dargonax for his own -- as a unlimited source of power from which he could feed. And so Zendarin made his way to the pit where Dargonax was being 'raised', and promptly began to feed him all of the energy the stolen staff he carried had in it. After Dargonax had devoured most of the remaining power from the staff, Zendarin told the dragon that it was time, and that Dargonax would obey his will in all things.
Dargonax laughed. This was not the expected response. Then he promptly tried to eat the blood elf, who abruptly vanished. Sintharia called her son to her, pleased at the gargantuan size of her 'child' and chastising him when he tried to eat Rhonin, who'd shown up looking for his wife Vereesa. When Dargonax tried to protest, she used the powers of the Demon Soul to subdue him -- powers that Rhonin immediately recognized.
Meanwhile, Zendarin appeared at the Demon Soul, intent on using it to control the dragons, since Dargonax obviously wasn't going to play nice. Vereesa found him there and as the two fought, she caught sight of the Demon Soul and grasped at the faint hope of an improbable idea. As she wrestled with Zendarin, she managed to twist the naaru-blessed staff and strike it against the Demon Soul. Since the staff itself was not Azerothian in origin, it and the Demon Soul simply shattered, and the released powers of the Demon Soul did what Dargonax could not, and had Zendarin for a tasty snack.
Dargonax and Zzeraku burst forth into the air above Grim Batol, locked in battle -- but Zzeraku was no match for the twilight dragon. Iridi attempted to funnel all the power she and the staff she carried into the nether drake, but Zzeraku was absorbed by the twilight dragon. In a last ditch effort, Korialstrasz fed Dragonax the last shard of the now utterly destroyed Demon Soul. That, combined with the energy of the nether drake he'd just absorbed were more than enough, and Dargonax detonated, taking Sintharia with him.
The last of the Twilight dragons was dead.
...sort of. Beneath Grim Batol, way, way down, Deathwing toyed with the eggs he'd taken from Sintharia's chambers. Scores upon scores of twilight eggs -- his mate had been quite the useful puppet for a time, and now that he'd had a chance to watch her mistakes, he would be certain not to make any of his own...
The Twilight dragons are far from dead, as evidenced in Wrath of the Lich King. The black dragonflight has adopted them for their own, using the Obsidian Sanctum as a hatching ground for the eggs Deathwing stole from Sintharia's experiments. The eggs are watched over by Sartharion, and players are charged to kill not only the black drake, but three Twilight drakes who are assisting Sartharion with the egg factory -- Vesperon, Shadron, and Tenebron.
Will Deathwing succeed? Not if players have anything to say about it, and the upcoming expansion should shed more light on the Twilight flight and Deathwing's ultimate plans for them.