The Vanishing Realms

The Dragon Council has sent forth a summons for all dragons and riders to assemble at Dragon Hall at once. Some of the realms visited by way of the Mist Trail for countless cycles have vanished.
Sixty cycles later, Storm Rider or Rogue Dragon or Dragon of Fire and Ice, at the request of a friend, has located Rider Dorma, the untrained, and brought her to the Dragon Hall to appear before the council. Then at the request of the council, the two set off to seek the reason for the vanishing realms.
As they travel, they meet with old friends of the dragon, as well as making some new ones. As they travel, they find they must fight tones and dragon tones, created by the dark arts of the blood and crimson sorcerers and a host of monsters out of nightmares.
Finally, they return to Cormac Realm to stand before the Dragon Hall with the other dragons and riders in a great battle to decide the fates of all the realms.