I am a little fuzzy on some of the details on this story, including whether it was a short story or full-length novella. What I do remember is that the protagonist is an adult male. He hails from a nearby village, in a society or world that is rather primitive, and technologically backwards. I recall the man being rather superstitious and/or fearful of certain taboos as well. IIRC, one of his fears is the mountain or large hill he is currently hunting on - something about legends about the place or the like.
As it turns out, there is something more to the hill than first appears. The man either stumbles upon, or is captured by, a group of people operating in a hidden base inside the hill. I remember these people as being human like the protagonist, but much more advanced. The protagonist may or may not be the first to be captured, but he comes the object of their intentions.
The man is physically augmented by his captors. His muscle mass and density are chemically enhanced. The vulnerable points of his body are protected with surgically-implanted plastic sheaths. He is trained in all manner of combat skills. The protagonist is also educated by the advanced humans, taught science and medicine and other disciplines.
When the physical and educational training is complete, the protagonist is tasked with carrying out a mission among his own people. I cannot for the life of me remember what it is he was supposed to do, but it sticks in my mind it is a benevolent one.
The protagonist is released from the secret base and returns to his people. It's possible the physical augmentation has made him unrecognizable as his former self, and may be initially seen as a stranger. I do not recall much more of the story, except for the climax.
The protagonist is faced off against another man, who is (IIRC) both the local chieftain/ruler, and a former friend of the protagonists. The two face off in a dueling circle, where the two must fight, and none are allowed to interfere. The fight is not necessarily to the death, as best as I can recall.
In the course of the fight, the protagonist accidentally shatters the Adam's Apple of the chieftain, causing him to collapse and begin to choke to death. The protagonist grabs a dagger and leaps on the fallen chieftain. The other tribesmen, realizing the chieftain is doomed, and seeing the protagonist's actions as cruel and unnecessary, leap on the protagonist to stop him. The protagonist is able to throw them off (several times) long enough to cut a hole in the chieftain's trachea. He blows into it several times, in order to get the ruler breathing again.
The story closes with the very much alive chieftain, now with a breathing tube installed, thanking the protagonist for saving his life. I do not recall what the protagonist does then (i.e. I don't know if he stays among his people, or if he returns to the camp with the advanced humans).
No idea really when it was published. I read it in the 1980s, so no later than that. Possibly quite a bit earlier.