I only remember the black serpent's host had a younger who was given the same name (and looked exactly the same) because the younger brother was meant to be a sacrifice for a curse.
I had to read back. They were adopted brothers, he was brought in to be a curse bearer for the older brother, and got reborn/now possessing a new body.
Such a long chapter and I understood nearly nothing. Just that these two are/were brothers. Fine. Now let me see them fuck