The Dawning Light by Robert Silverberg

On the planet Nidor the ties of tradition and stability were coming undone. Unprecedented things kept happening, and although nothing could be traced to the Earthmen, it was clear that the troubles had begun with their arrival.

Norvis peRahn Brajjyd, unjustly expelled from the Earthmen's school, believed he knew why: The Earthmen were not emissaries of the Great Light, as they claimed, but demons from the Outer Darkness.

He vowed that he would stop at nothing, even the most unthinkable deed, to break the hold that the aliens had on the people and the Council of Elders.