The Dao Fetus that Fuxi spoke of was, in truth, a vessel for storing karma. It would store every action Si Ren took deep within the body.
Once mature, it would detach from Si Ren's body, taking all that karma with it.
Naturally, this Dao Fetus could also be viewed as Si Ren's substitute. He might become an entirely new life form, but the fetus would be the one held accountable for all that karma.
Chapter 1587