
I can't read Korean but here's what I understood:
The spoilers are referring to the last chapters, which are still about Sunbae and Hosik but in ancient Korea or Korea in the past. In that time, Sunbae, I assumed, is a royalty and Hosik is like a commoner. They fell in love too but I think because Sunbae was obligated or something to produce an heir, I think their union was not possible or not allowed but they really do love each other.
In the last chapter, Sunbae has to go on a journey for some political/royalty stuff but when Sunbae returned, it turns out that Hosik is already dying of some sort of disease. Sunbae hurriedly goes to the dying Hosik and tearfully beg him to probably not go or something. Sunbae is ABSOLUTELY DEVASTATED. Hosik said a few words to Sunbae before finally succumbing to his disease, leaving the devastated Sunbae.
It turns out that the current Hosik and Sunbae we have right now is the incarnation of those two and this time, they finally get together and become a happy family.

It's pretty long, actually. Past sunbae and Past Hosik met when they were kids and Hosik was Sunbae's servant. Sunbae was actually married to a woman and had a child but all the while he maintained a relationship with Hosik because he's the only one he's ever loved. But it's not an equal relationship because they're Master/Servant.
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Actually I dont understand everyone's spoilers.
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So is this something like sci-fi/time travel where hosik gets reincarnated or something?
Cuz I'm honestly lost o.o