At some point in the past, my grandfather let me read a book featuring tribes at war (I think it was 6). Each tribe had some carefully guarded unique power, one I remember was rapid regeneration. This regeneration was so fast that anything short if cutting of their head was non-fatal.
I recall some major incident where the main character, who is one of these regenerators, is cut in half and BOTH halves regenerate. The story of course concludes with a confrontation between the MC and his doppelganger.
I also recall there were individuals who could shape-shift, or make themselves appear as other people, and they were infiltrating all the tribes and inspiring conflict.
The MC was a prince or heir of his tribe, and he killed his younger brother by accident. He injured him in a way his regeneration should have fixed, but it wasn't his brother, it was one of the shape-shifters.