
Jaheons family gives mugo a sacrifice in return for a wish. They created mugo solely to take advantage of him, and mugo wants revenge on his family. The old man mugo views Jaheon as a hindrance and wants to let him go while the other mugos (the monster and the main one) joined forces to keep Jaheon and protect him.

So that’s a part of the story that still hasn’t been explained, you can reread it now that there aren’t many chapters in mind (I did this lol) and keep these things in mind to look out for
1. Look for a pot, Mugo mentioned being created in it i believe
2. Regarding the pot I also believe they mentioned a body was cut into pieces
3. When they are having a 3som and the smart Mugo is looking, a ‘spirit?’ appears and says something like “oh what a god you are” (idk)
4. When jaheon wakes up from the 3som I believe, there is a gray hand that looks to be the possible spirit
5. When jerk Mugo was in a hallway, the past spirits of them are seen talking to each saying something like “I have always been yours young master” and then jerk mugo says “no, it can’t be”
BASICALLY the lore of their past hasn’t been revealed, it’s something that is slowly being uncovered. But it’s for sure that they had something to do together in their past lives. Kind of like Dawn of the Dragon I believe
I hope this somewhat helps (〜 ̄△ ̄)〜

From the pictures/artistic hints that were given they may explain it through folklore Japanese/Chinese practice of kodoku, where venomous snakes (and other creatures) were placed in a jar to fight, with the survivor's potent fluids used for a lethal poison. In other manga gods are created thanks to (a) having many believers, (b) strong emotions; but these are more like my hypothesis, and it still doesn't explain the 3 personalities clashing into one body.
I don't fully get the whole plot still