In my country, we are split down the center by an intensely large mountain range. Us desert people live on the eastern half of the country, while on the other side of the mountain range the forest people live. Although we are separate for most of the year, every winter we are forced by brutal winter storms into the mountain caves.
200 years ago, a third group of people, the ones who built out the caves and made the central trade tunnel from east to west, suddenly disappeared. Just as they started to fade from memory, there has been an increase in reports of attacks on caravans journeying through the tunnels. It has been suggested that the attacks have been more brutal then the standard highway pirate fare, only a single survivor had managed to escape but without water he died in the tunnels.
It is becoming clear that the attacks were carried out by the lost people, but we have little to no idea of what they even look like. How would 10 generations of living underground with no light, sourcing all foods from things grown and found underground, and a societal focus on digging tunnels change the human body?
Edit: To specify I am not expecting much evolutionary change. What I am expecting is that living in those conditions will result in a body that does not look like a surface dweller. Most obvious would be the pale skin. Not an evolutionary trait, but a product of the environment that changes the population's body. The lost people have developed massive fungi that reproduce quickly and have a high caloric value, so starvation is not an issue for the lost people still associated with the main pack. And technology is equal to that of 14th century Europe, with significantly more metallurgy knowledge. The best answer will focus on physical changes based on living in a cave and less on the evolution of the species.