If sun shan was the host of the ghost monster. And he saw the gost monster when his auntie was alive then why is it coming after the other dude too, have you guys ever wonder about that. And plus sun shan auntie can see ghost ever since she was little, why she didn't see the ghost monster.
Sorry this is long but I think this is what happened:
Hwan likes to eat people with high spiritual energy. He found Auntie and Sun Shan, decided he wanted to eat Auntie first.
Maybe he didn't show himself to her or she just sees so many ghosts every day that she didn't care about him.
After he ate her soul, he started haunting Sun Shan for his next meal. When he got possessed by Hwan, Sun Shan became the host.
Now Geumsun/Yama will try to kill Sun Shan because if you kill the host of a ghost, the ghost dies with them. So it's when they are most vulnerable.
Shit will go wrong, maybe Geumsun will injure Hwan and Sun Shan's soul-body will go to the underworld, which will make him lose his memory. Geumsun will take care of Sun Shan, who is now named "San" since he doesn't remember who he is.
San will meet Eunjae, a human with spiritual energy that he likes to have sex with :) . From Geumsun and Yama's perspective, the noth of them are bait to capture/kill Hwan since he wants to eat them to heal and power up.
Fast forward into the future and you're caught up with what's happening now: Geumsun is finally explaining to our main characters why Hwan wants to eat them both
Do anybody gots some books just like this