These are comments from the username Nogi from comix that I found very helpful in understanding what is happening (mentioned by dooshik_right_toe below):
After I reread it multiple times, I think I now understand the plot . This contain spoiler
Muni a servant to Do Il In, obsessed with his master that he always stalked him. His master frustrated by Muni, planned to kill him by abandoning him in a pit that meant to caught a shape shifting monster snake. He left Muni inside the hole, and rain start to flood the hole but Muni survived but left in a vegetative state. That's when the master fall in love with him because Muni didn't respond to him (he has non-con fetish). Unfortunately, Muni was dying so he seek out shaman to do a soul transfer ritual with the snake (cause it can shapeshift into human)
Yooran's ancestor (Kiyoon female colleague) who is a shaman agree to help. Master Il In told the shaman that he doesn't want part of Muni that attached to him, he want Muni that act like he is in vegetative state (or someone who refuse him like our MC kiyoon). The shaman agreed but need an item to store the unwanted part of the soul, master Il In gave her the bookmark
Please remember, that this unwanted part of soul was terribly obsessed with master Il In.
During the soul transferring process, Muni's body was cut open and his blood was feed to the snake. A lot of servant died in the process. Unfortunately the ritual failed. Master Il In sustained an injury his left eye leaving claw mark scar. He confronted the shaman and she performed a ritual so that master Il In and Muni will be reborn together. One part of Muni's soul (that was stores in the a bookmark) became a vengeful spirit because he can't go to afterlife and latch on the shaman and her ancestors, as a karma for her evil deed.
Fast forward
The second ritual succeed, Muni was the reincarnation of our MC kiyoon and master Il In was reincarnated as Manager Jung Yitae.
Kiyoon can see ghost since he was a child but he made a promise to pretend he didn't know anything or fake ignorance. The person he made that promise turn out to be the other half soul that was left inside the body. In contrast with the unwanted soul (inside the bookmark), this one hates master Il In and it remember everything he has done in the past and all the suffering Muni felt during the ritual.
The soul that was left in the bookmark, I will call him Muni for easier read. He became a ghost that follows Yooran's family. That's how he met Manager Jung yitae. Yitae looks so much like Master Il In and become obsessed him Yitae. When Yitae met Kiyoon, he fell for him at first sight. Muni got jealous and possessed Yitae's body. When he touched Jumin's hand, he accidentally sucked his life force (sm like that). He then continues use Jumin that in love to Yitae (mainly because Jumin looked like him in his past life, while Kiyoon looks exactly like Master Il In's type.
Bc Yitae don't remember his past as Il In, he created his own Master Il In (I will call him ghost) complete with his memory of Il In (except the part where master Il In feel in love with him after he got into coma) That ghost is the one who kept bothering Kiyoon after he filmed the ghost on his phone The ghost was attracted to Kiyoon which set off Muni. Muni monitored Kiyoon through his phone btw
Bunch of nsfws with the yitae ghost happened that you can read, and Kiyoon started to have dream about his past life. After he remembers that he is the reincarnation of Muni, he met a shaman ( yooran's grandma). When he arrived Muni had possessed yooran and the grandma went missing. Muni start a explaining his grieve over his life and only want him to be love by the manager or the reincarnation of master Il In.
Then we saw a scene of manager yitae driving, at that time, the original soul of master Il In emerge, he then goes to seek Kiyoon and grape him until morning.
Kiyoon woke up by the friction saw the ghost figure staring at them having smex and thing turn into 3p The ghost recognize master Il In and it set Il In mood off and they vanish and left Kiyoon in disheveled state with no after care.
Bunch of things happened, master Il In explain to Kiyoon that he only love the current him and hate Muni. Kiyoon got offended because he is also Muni. Then Muni's soul that hate Master Il In possessed Kiyoon and stabbed Il In Muni got jealous cause master Il In loves only Kiyoon but treat him like servant, so he wanted to kill Kiyoon and take over his body.
The ghost that Muni create fell in love with Kiyoon and want him to himself which make master Il In jealous and want to eliminate him.
Bunch of things happened that I can't understand cause I don't know Korean, but master Il In end up strangled by the vegetative state Muni that has merge with the snake, manager Yitae and the ghost become one and Kiyoon and his other part of soul also become one... And the new raw still didn't uploaded so that's it.
I hope you can understand this, there's lot of plot twist in this story and honestly it's enjoyable to read.(｡•︿•｡)
These are comments from the username Nogi from comix that I found very helpful in understanding what is happening (mentioned by dooshik_right_toe below):
After I reread it multiple times, I think I now understand the plot . This contain spoiler
Muni a servant to Do Il In, obsessed with his master that he always stalked him. His master frustrated by Muni, planned to kill him by abandoning him in a pit that meant to caught a shape shifting monster snake. He left Muni inside the hole, and rain start to flood the hole but Muni survived but left in a vegetative state. That's when the master fall in love with him because Muni didn't respond to him (he has non-con fetish). Unfortunately, Muni was dying so he seek out shaman to do a soul transfer ritual with the snake (cause it can shapeshift into human)
Yooran's ancestor (Kiyoon female colleague) who is a shaman agree to help. Master Il In told the shaman that he doesn't want part of Muni that attached to him, he want Muni that act like he is in vegetative state (or someone who refuse him like our MC kiyoon). The shaman agreed but need an item to store the unwanted part of the soul, master Il In gave her the bookmark
Please remember, that this unwanted part of soul was terribly obsessed with master Il In.
During the soul transferring process, Muni's body was cut open and his blood was feed to the snake. A lot of servant died in the process. Unfortunately the ritual failed. Master Il In sustained an injury his left eye leaving claw mark scar. He confronted the shaman and she performed a ritual so that master Il In and Muni will be reborn together. One part of Muni's soul (that was stores in the a bookmark) became a vengeful spirit because he can't go to afterlife and latch on the shaman and her ancestors, as a karma for her evil deed.
Fast forward
The second ritual succeed, Muni was the reincarnation of our MC kiyoon and master Il In was reincarnated as Manager Jung Yitae.
Kiyoon can see ghost since he was a child but he made a promise to pretend he didn't know anything or fake ignorance. The person he made that promise turn out to be the other half soul that was left inside the body. In contrast with the unwanted soul (inside the bookmark), this one hates master Il In and it remember everything he has done in the past and all the suffering Muni felt during the ritual.
The soul that was left in the bookmark, I will call him Muni for easier read. He became a ghost that follows Yooran's family. That's how he met Manager Jung yitae.
Yitae looks so much like Master Il In and become obsessed him Yitae. When Yitae met Kiyoon, he fell for him at first sight. Muni got jealous and possessed Yitae's body. When he touched Jumin's hand, he accidentally sucked his life force (sm like that). He then continues use Jumin that in love to Yitae (mainly because Jumin looked like him in his past life, while Kiyoon looks exactly like Master Il In's type.
Bc Yitae don't remember his past as Il In, he created his own Master Il In (I will call him ghost) complete with his memory of Il In (except the part where master Il In feel in love with him after he got into coma)
That ghost is the one who kept bothering Kiyoon after he filmed the ghost on his phone
The ghost was attracted to Kiyoon which set off Muni. Muni monitored Kiyoon through his phone btw
Bunch of nsfws with the yitae ghost happened that you can read, and Kiyoon started to have dream about his past life. After he remembers that he is the reincarnation of Muni, he met a shaman ( yooran's grandma). When he arrived Muni had possessed yooran and the grandma went missing. Muni start a explaining his grieve over his life and only want him to be love by the manager or the reincarnation of master Il In.
Then we saw a scene of manager yitae driving, at that time, the original soul of master Il In emerge, he then goes to seek Kiyoon and grape him until morning.
Kiyoon woke up by the friction saw the ghost figure staring at them having smex and thing turn into 3p The ghost recognize master Il In and it set Il In mood off and they vanish and left Kiyoon in disheveled state with no after care.
Bunch of things happened, master Il In explain to Kiyoon that he only love the current him and hate Muni. Kiyoon got offended because he is also Muni. Then Muni's soul that hate Master Il In possessed Kiyoon and stabbed Il In
Muni got jealous cause master Il In loves only Kiyoon but treat him like servant, so he wanted to kill Kiyoon and take over his body.
The ghost that Muni create fell in love with Kiyoon and want him to himself which make master Il In jealous and want to eliminate him.
Bunch of things happened that I can't understand cause I don't know Korean, but master Il In end up strangled by the vegetative state Muni that has merge with the snake, manager Yitae and the ghost become one and Kiyoon and his other part of soul also become one... And the new raw still didn't uploaded so that's it.
I hope you can understand this, there's lot of plot twist in this story and honestly it's enjoyable to read.(｡•︿•｡)