Zendarin Windrunner was the blood elf cousin of the Windrunner sisters, who allied himself with the black dragon Sinestra in an attempt to create the twilight dragonflight. As a child Zendarin dreamed of rising among the ranks of the high elves, and become a strong and powerful noble. When the Sunwell was destroyed, Zendarin's hunger for magic caused him to become a blood elf. Zendarin unsuccessfully tried to kidnap Rhonin and Vereesa's twin sons, Galdarin and Giramar, because he thought the offspring of a high elf and a human wizard would be powerful. Zendarin despised humans, thinking them as little more than animals, and didn't put much higher opinion of Vereesa (and likely also of Alleria), who had married and had children with a human.
Though Zendarin helped Sinestra create the twilight flight in the promise of being shown new sources of magical energy to sate his addiction, he had his own reasons for allying with her: he secretly wished to take control of Dargonax as a magic source and his own personal dragon, and tried to do so with a final feeding to make him his slave. ( Sinestra had foreseen all however, and simply made Zendarin's betrayal a part of her plans, which resulted in his failure and escape from the hungry twilight dragon. Zendarin then sought to steal the )Demon Soul (possibly as an alternate magic source having failed to take Dargonax, or as a secondary objective), the foul creation of Deathwing that had been destroyed by Rhonin years earlier. He would have pulled it off if not for Vereesa's intervention, which resulted in the destruction of the Demon Soul — and of Zendarin himself — when the stolen staff of the Naaru touched the artifact.