A doreki is a device made from mechanics and soil. It connects earth to flesh, and makes a body stronger, and more resistant to pain.
Soil is infused with the spirits of the dead. It's then placed in the doreki, which converts it to vitality. Oguna gets his superhuman abilities from this pipe. It connects clay/soil with flesh.
The doreki has something called a soul vessel that connects the human body to the doreki. It's like a lifeline, and if it is broken, it means death, unless someone capable can repair it. To do so, spirit clay is needed. Spirit clay is a refined form of soil infused with human souls. Usually, it's blood soaked earth, but in Oguna's case, it's earth that nurtured the holy tree in Kizumi, deep within the forests of Jagara.
Oguna's father had his memories sealed and he had false memories until 1~2 years ago. Like that the spirit pipe was in his body because he had a weak body and that his mother died giving birth to him. The pipe was embedded in him because the emperor wanted to make a strong assassin out of him, and his mother was killed when the emperor's people took him from her against her will.
A person who has a doreki in their body cannot float, and will sink like a stone, because they are part soil.