People of the Black Sun (People of the Longhouse #4; North America's Forgotten Past #20)

Dekanawida has become known as “The Sky Messenger,” a prophet of immense power, and Hiawento is his Speaker. Thousands now believe in the Great Law of Peace and have joined the League. But they are still being harassed by marauding warriors from the People of the Mountain who steadfastly refuse to adopt the Great Law.
Dekanawida has prophesied destruction if the warfare continues. As one by one, portents start coming true, Dekanawida has one last chance to convince the People of the Mountain to join the League and save their world from utter destruction.