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I believe u r right in the last paragraph. Seeing the latest raws for the next season (cos it already started), Muheun saw something when he touched Seungjoo's hand when he was a baby. And it stated in this season that Muheun can see the future. (▰˘◡˘▰) So probably they r soulmates, thats why he is so focused in protecting Seungjoo even when Seungjoo's partner should be Muryeong (i forgot the term but Seungjoo mentioned it in the earlier chap).
I did not read the novel. This is all from what is shown to us. The MC belonged to a rich family that burned a holy mountain and fled with the offering hundred of years ago. Why they burned the mountain no one really knows but recent chapter suggested they burned it to save a friend. So the priest cursed them to be have misfortunes or magbet for bad luck until their family cease to exist. The ML belonged to a family of exorcist that is very close to the MC family. I think their ancestors were in love with each other. Anyway, the ML's ancestors vowed that her descendants will always protect the descendants of the MCs family.
Now comes the present in which MC had two older siblings but is now deceased. The ML is the eldest of the three in his family. So it seems MLs family is really trying to do a 1:1 ratio for protection but sadly failed since the older siblings died.
The interesting part here is that the ML is so obsessed with the MC starting when he was still a baby. Like there is something the ML is hiding, I also think this obsession is the reason why the 2 older siblings died. That's my theory though, with how things are playing I wouldn't even be shock if he knew that the MC might be his soulmate.