For tens of thousands of years, long before Flamekeep or even Thrane, the Tamor Hills have been home to wyverns. Most of these are typical wyverns, incapable of speech. However, these cliffs are also home to an exceptional strain of wyvern that is more intelligent and capable of much greater sizes.
The early settlers of the region formed an alliance with these elder wyverns, and many of Daskara’s (which would eventually become Thrane) rulers had wyverns as advisors. During the Year of Blood and Fire, Tira Miron recruited the elder wyverns to her cause. The elder wyvern Ashtarax carried her into battle against Bel Shalor.
In the aftermath of Tira's sacrifice, the wyverns of the Tamor Hills adopted the faith of the Silver Flame and chose to protect Flamekeep as their sacred duty. They have relatively little concept of the rest of Khorvaire, and they don't care to learn. The wyverns serve the Church because they believe that the Church serves the Voice in the Flame, that Tira Miron still guides them.
Thrane fielded wyverns in battle during the war, which gave them aerial superiority only rivalled by Aundair. In addition to normal wyverns, some of these elder wyverns also joined. These were members of the Order of Ashtarax, a sub-order of the Knights Templar dedicated to them and their riders, named after the wyvern that carried Tira Miron into battle.
Elder wyverns were terrifying presences on the battlefield. Not only they could carry entire squads into battle, but they were intelligent enough to engage in complicated tactics beyond their lesser kin. Some of these elder wyverns could even channel the divine power of the Silver Flame.