And yet the Unifier planted the seeds of a deeper unity. Not since the collapse of the Coalition had the successors of mankind recalled that their ancestors had shared the same warm pond. After ten thousand more years that unity found a common cause.
Mankind’s hard-won Galaxy was a mere tidal pool of muddy light, while all around alien cultures commanded a wider ocean. Now those immense spaces became an arena for a new war. As in the time of the Unifier, disparate human types were thrown into the conflict; new sub-species were even bred specifically to serve as weapons.
This war continued in various forms for a hundred thousand years. In the end, like the Unifier, mankind was defeated by the sheer scale of the arena – and by time, which erodes all human purposes.
-Resplendent, "Between Worlds"