At first the story is hard to understand because there's so many of names and other elements. As the story goes it become clearer.
So here's some synopsis that might help you guys understand it.
It started with the legend about demon that steals the mirror that was left to the humans after the immortals goes to heaven. But it turned out that the story told to the humans is wrong. Kangchul who was one of the four generals who stole the mirror told them that the human tricked them so that they can monopolize the mirror for themselves and sealed the demon underground. So Geusendeu (?) The other general told them to steal it and kept it inside their body or weapons. After that the demons live in the above ground.
Hojung who was one of the generals picked up the child after the situation is peaceful. And that child is the minister taeryung. Taeryung fall in love with Hojung. And hojung just noticed that taeryung is not human. He's half breed. And his parents was killed by the demons that come above ground. He felt guilty for some reason so decided to sealed the generals and give back the mirror to the human so that the sun will shine again.
In the present taeryung broke the seal and the general with its shard of the mirror broke free again. And that's our timeline.
I think that's the gist of it.
Sure this is a bit boring and the way the author told the story is not that good. I got bored reading it. But I like how the problem between the couple is very realistic. The conflict in this webtoon is very common and usually what happen in real life. So I have to give a thumbs up for that. Usually other webtoon will just have lovey dovey moment. Or third wheel and etc. If only the plot is not rushing this will be very good.
Next week is gonna be a major angst... I wish time will fly...