Poul Anderson 
Day of Their Return 
A Flandry Book

Back under the thumb of the Terran Empire after leading their planetary sector in an almost successful war against Imperial rule, Commissioner Desai saw real trouble brewing this time. A strange, fanatical movement was spreading like wildfire: there were rumours of the return of the fabled Elder Race: the Firstling, leader-elect of the planet, was on the run and hiding from Imperialist retribution. And off-planet agents from the Ythrian Domain and Mersia, Terra’s ancient foe, were abroad in the land. Unless Commissioner Desai could damp the fuse of rebellion, the universe would begin its terrifying descent into the Long Night . . .