[Winters_Sorrow]

Nightfall - good choice!

My suggestion would be "The Praxis" by Walter Jon Williams - quick precis below:

The empire of the Shaa lasted 10,000 years. Years of terror, infinite violence and oppressive, brutal order. Now the Shaa are no more, but the terror and violence are only beginning...
And so began the war between the Naxids, oldest client race of the Shaa, who believed themselves inheritors of the empire, and a frail alliance of other races, including humanity. As the empire expanded, through wormholes which crossed both space and time, the Shaa formulated The Praxis, a harsh ethic which they imposed on those they conquered. But it could also serve as a cloak for insurrection...