A Knight of Ghosts and Shadows — Poul Anderson

Raconteur, bon vivant, troubleshooter for the decaying Terran Empire, Dominic Falndry doesn't crave further danger in the service of galactic unity.

But duty calls, so it's back to the spaceways for the most elegant Special Agent is a hundred star systems--straight into the well-laid plans of his lifelong enemy Aycharaych.

Win or lose, though, the long night of human civilization is coming and Flandry knows it. How many more battles can he stand to win in a losing cause? And how many planets will die meanwhile?