Complex sentence: A comrade later showed him the bodies being burnt in a pit, where a "Kapo" there told him details of the burning, such as how gases developed in the body, seemed to make the burning corpses move.
Split into simpler sentences: A comrade later showed him the bodies being burnt in a pit. A "Kapo" there told him details of the burning, such as how gases developed in the body and made the burning corpses move.