I tried my best to explain but to be honest I’m not entirely sure so if there’s anything missing or wrong, feel free to comment!!
San was a human. He had powerful spiritual sight and a powerful spirit, this means his body-soul gap was very wide which was tempting for spirits. We saw that his mum was stressed because of this and then left him with his aunt who can also see spirits/ghosts. Then San meets Hwan. After Hwan was injured by Geumsun, he was recovering and now he’s back.
San is excellent prey for Hwan because he has such strong energy. For Hwan to consume, he had to possess his victims but people with strong spiritual energy can also resist him with mental power. That’s why he gets rid of whatever is sustaining them (eg family, friends) to break them down.
Obviously, Geumsun and Yama want to get rid of Hwan so Geumsun goes to find San. He can get rid of Hwan if he is possessing San’s body. “When the host dies, so does the monster” but if he kills Hwan in San’s body then San’s soul will disappear along with the monster. However, Geumsun couldn’t kill him and Hwan left San’s body I think?
After that, Geumsun needed a plan. San could not continue to stay in the human world because he will eventually be consumed by Hwan.
His plan was-
Send San to underworld alive but erase everything about him on earth and wipe his memories. This way Hwan might forget about him too.
Then send him to mortal world disguised as a “goblin” (he will be covered in Geumsun’s energy). When the time is right, Geumsun will remove his energy and Hwan will see San as a human and be tempted to possess him again. This gives him another chance to kill him.
The thing about 704-
In Chap 54, yama gives 704 a note with an “unaccounted death” and he wants a reaper to go collect it. 704 sees that the death was already 10 hours ago but does not think much about it as the records had been a mess lately due to Hwan killing many humans. I’m guessing he went and just collected San even though he was not dead yet ? So he feels guilty??
did he already obtain it at the beginning of chapter 1?? I have no brain cells to make up theories as to what happened but ..yea
I think ch 1 was a flash forward or the memories of the mc that wrote the diary.