Here's a tidbit of background for my fifth race. This is all you're going to get:
(NB: The "Z'Raki" are the Shivans)
We were the first, the creators. We were the masters of the galaxy, nay, the universe. We so jealously guarded Our superiority that We became enraged when We saw weaker species beginning to rise towards Our level. We created a guardian, the Z'Raki, an unstoppable monster only We could control. We let it loose to make sure that no one would ever be able to challenge Us. They ripped through the pretenders to Our throne at a terrifying rate. However, one species fought back. No mater. Our creations tore at them again and again until at last, they were defeated.
But then, two races arose, races like no other. They fought each other with a ferocity never seen before except in Our Z'Raki servants until their technological advancement had reached a point at which We decided to exterminate them. Our servants viciously attacked them, but they would not fall. They fought back with a vengeance, stealing Our creations' technology and using it against them. We decided to wipe out their homeworlds. One world fell, but they followed the Z'Rraki and destroyed their mothership.
We were furious. Thirty-two years later, we sent Our guardians back to finish their job. The two races, now friends, counter-attacked, and the war became a dead heat. We destroyed their flagship at Capella, but still the two races pressed on. The Z'Raki showed their might by tearing the Capella star apart, but then...they stopped. They had abducted one of the aliens, who intrigued the Z'Raki. They actually allied with one of the two inferior races and then all but destroyed the other! Then their old nemesis, who they thought they had destroyed thousands of years ago, returned, but the Z'Raki and their allies drove them out but did not destroy them utterly.
We wondered why the Z'Raki refused to obey Our directives, and suddenly We knew: they no longer feared Us. They believed they were supreme in the universe. We knew that it would only be a century or so before they rose to Our level, so We decided to exterminate them and their allies. We will teach them what it means to defy Us.