First, there came the Great War Among the Gods. The Pantheon was split into two factions, each led by one half of the recently split Io. On one side was Bahamut, the other Tiamat. When the dust settled, the entire Pantheon was nearly wiped out, each Deity replaced by his or her Exarch. Bahamut had fallen, and Tiamat emerged victorious without any Deity powerful enough to challenge her.
For the past few centuries, Tiamat has ruled through her Council of Five from the heart of her Empire, with Dragons and their Dragonborn ruling oppressively from their places of power, keeping the other races in check by any means necessary.