Ambush planet

The Old Litani escaped the planet Vassa just before its civilization was obliterated by wars. Now, after millenia, they're back and in control, using their highly advanced technology to regulate the minds and actions of the New Litani, animal-like humanoid inhabitants. Anthropologist-linguist Brian McCann's mission also bringshim to the fertile, green planetto guide settlers fleeing an overcrowded Earth. Instead, he discovers the Old Litani have thrown up a defense system which will cause the gigantic colony ships to crash. Before he can warn Earth, McCann is captured by humanoids who have eluded the power of the old ones, and he is given a choice: his severed head on a bloody platter, or join in a life-and-death struggle to overthrow their brutal oppressors.