literally the same. bc this webtoon is the adaptation. https://www.novelupdates.com/series/unseen-immortal-of-three-hundred-years/
Spoiler alert
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Yes, he's the actual owner of the body and never transmigrated. The place where he thought he came from, it was actually a dream. He was once a heavenly figure and very important one in fact. He's the one who judges who will get exiled from the heavens... It will be known from ringing the bell. ML and MC has a long history. The last update I've read in the novel, they were in some sort of illusion of the past of where MC's demonic kingdom resides so I don't know why he became a demon king. Also despite being a demon king, demons goes through something similar to heat and ML still helps MC with that .
Funny tidbits is that MC doesn't like to hold anything like his sword so he makes ML hold it when he was still in the heavens. In the upcoming chapters, MC will be reunited with his bell which he let ML hold it but ML was somewhat careless about it and MC thought the bell would break so he went and took it back XD
I think I half finished a BL novel with the same name, I pretty sure this Manga is based off the BL novel but unfortunately like most Chinese manga they heavily censor the BL to the point where it just completely rewrites the OG story not to mention the translations leave much to be desired....(╯°Д °)╯╧╧