Ages ago, the kedamono regarded humans and their pure souls granted to them by the Lord of Light with jealousy. Before long, they developed lures and went after humans, wantonly claiming their souls for themselves. The Lord of Light grew angry, thus did he bring forth his faithful to fight a war against the kedamono.
Humans, of course, are no match for a kedamono. Not even an army may be enough. So the Lord of Light began to create artifacts to empower his soldiers. One type of these artifacts were called God's Chariot, and, quite unlike the horse-drawn cart you may be thinking of, these Chariots were statues fashioned in the shape of animals or humans, and whoever so occupied one could control it as though it were their own flesh. And through the Chariots, one human could be a kedamono's equal.
Yet, the Chariots were fashioned with conceptual hearts, organs that served as the Chariot's core. This gave the statue its power, and its nervous system, as well as gave it limited intelligence. Chariots could walk and swing a sword on their own, but this was nothing compared to what a Chariot could do when connected with its pilot. The "neural reins" connected conceptual heart to pilot soul, and as such, the two would become one.
Chariots did not have a soul of their own. That was for the pilot to provide. Yet, over countless years, with each kedamono felled, fragmented soul of the slain would build up in the Chariot's heart. Until one day, the soul became a whole false soul, and the Chariot itself became a kedamono. In so doing, it would absorb its pilot unthinkingly, draining away the pure soul it was attached to via its neural reins, killing their pilot.
It is almost romantic to think of a weapon of war and its pilot, the two sharing a soul, a heart, mind, and memories. The tragedy, then, of being unable to help yourself from devouring your pilot's soul from the inside out. Forming your personality out of their memories. Having their body, still attached to you via the neural reins, under your control—a reversal of up until now. Knowing that while you can manipulate them to pilot yourself, without a true soul, your full glory cannot be known again.
And so you must look for a new pilot, with a new soul.