The Empire was ruled by an emperor or empress, who were originally members of ruling houses, including the houses of Gadarel and Sorsha. Various oaths were taken for "the Empire". (AUDIO: The Burning Prince)
The Drashani Empire was technologically advanced, though anachronistic in some areas. They used laser longbows instead of guns, but they did have more traditional weapons like flare charges. Their spaceships travelled through warp streams and were equiped with forcewalls. (AUDIO: The Burning Prince)
The Empire was originally ruled by a mad emperor who was eventually deposed. The Houses of Gadarel and Sorsha both claimed control of the empire and went to war. At some point during the war, the planet Sharnax was overrun by Igress. After decades of fighting, Valdon Gadarel was killed in battle, leaving behind Princess Aliona as the sole heir. She was less warlike and planned to marry Prince Kylo of Sorsha, combining the houses and allowing them to rule together.
When Princess Aliona's ship was sent to meet with Prince Kylo, it was sabotaged and crashed onto Sharnax. A rescue mission was sent by Sorsha, with Prince Kylo coming along. The ship brought an Igress with them in order to learn their weaknesses. The Fifth Doctor accidentally landed in the ship and was captured as a spy. At this point, the Igress was let loose by the actual saboteur. The Igress was only killed when Captain Shira blew a hole in the ship, causing it to crash onto Sharnax. (AUDIO: The Burning Prince)
Much later, there was the Succession of Blood, when Empress Cheni took the throne. After 15 years of peace, the Drashani Empire was attacked by the Wrath. This war lasted for 5 years, with the Drashani Empire losing. (AUDIO: The Acheron Pulse)