Adding additional theories related to the special episode tellung bits about the story.
The book Kodoku legend. Pretty sure that this was the curse used to poison Mugo.
I only looked at wikipedia, but Kodoku is where a bunch of insects or snakes are placed in a jar and they fight to death there. The insects/creatures that are still alive were fed to the criminals, poisoning them. You can keep one of the remaining insects to get luck.
Past life Jaheon might be a scholar, and maybe studying ancient curses. And knows Kodoku, and the people might used his studies to kill Lee Muhan.
And also TIL Mugo is a curse. Search on google k-occult Mugo. I think Lee Muhan is a very important person who fell in love with the teacher/scholar.
What if the baby got sick because he shared some powers to his dad. Oh noes.
WAIT FR