And they skin his back and tattoo him, and he basically goes crazy from the pain and isolation and wanders about
This strikes me as The Descent by Jeff Long.
Humanity discovers the remnants of a vast ancient empire underground, with the peoples consisting of a very devolved alternative to homo sapiens that humanity calls homo hadalis - or hadals in general usage.
The person captured is a butterfly collector who ventures underground after a rare butterfly, is caught by the hadals who enslave him, and “torture” him by tattooing and flaying his back in the style of his butterfly, causing him to go insane. He is allowed to wander the hadals world as a work of art.
There are also two separate soldier characters in the book, both forming part of the main storyline - one is also captured by the hadals and enslaved for a period, the other one is disfigured through an encounter with hadals.