The Sacred Hills by Don Coldsmith (Spanish Bit Saga #8)

From the North rides a savage tribe of warriors who kill for pleasure. When the Blue Paints attack the People, they murder the wife of Looks Far, medicine man to the People, and into his hands falls the People's only hope. He must seek the help of the People's old enemies, the Head Splitters.

Paperback, 1988

Goodreads: 4.25