The Aesir were once the most powerful force in the galaxy. They ruled over each system with a solid grip, but were more or less benevolent. Their only vice was halting the technological progression of races after a certain period, not wanting any possible rivals.
When the Humans first found this galaxy, they and the Aesir had a moment of caution. Each side sized up the other, until they realized they were more or less technological equals and knew any outright war would end in disaster for them both. So instead, the Aesir welcomed the Humans with open arms and ruled the galaxy alongside their new found allies.
When the Humans died out and the Hume replaced them, the Aesir believed they would have found a new ally in this offspring race. Instead, they found an extremely manipulative enemy. The Hume made their way into all aspects of Aesir government and military, and attacked them from the inside out. The Hume made good use of the Aesir technology, efficiently wiping out the Aesir in the matter of a thousand years.
All that remains of the Aesir now are implementations of their most rudimentary technology which, compared to any technology, is still highly advanced. Much to the chagrin of the Hume, it turned out the Aesir hid the secrets of their most advanced technology purposefully. Though it could not save the Aesir in the end, it helped to ensure that the Hume could not follow through with their original intention of enslaving the galaxy.