Ok here's my theory. I think the Young Master did something to put Muni in a coma (trapping him in a pit or well to drown?) when they were both alive in the past, then regretted it.
MC is Muni reincarnated, but the ritual that was done had the Shaman lady pour the blood of the serpent monster into his mouth. Because Muni and the Young Master's souls were tied to paper, known to be fragile/intangible, their bond was not strong enough to bring Muni back from the coma. Instead, the soul of the serpent monster possessed Muni's body and became Muni. This is why he has the weird long neck thing going on, and why he insisted HE is Muni.
Meanwhile, the real Muni's soul was ousted and reincarnated into MC. The Young Master didn't know the serpent had taken over Muni's body and that's why he hates Muni now.
Young Master's soul is bound to fake Muni's by the bookmark, and somehow Manager got his hands on it and uses the Young Master's soul to get hoes? (Not sure how that all happened but based on the earlier chapters it seemed like he was aware he was possessed by the Young Master.)
