The Moon That Embraces the Sun

Yang Myung is gonna die in the end.
It's a rather heart-breaking moment where he chooses not to save himself.

How the sotry will end will it be a happy ending?

It depends on what you call a happy ending. For the OTP, yes. Hwon finds out about the conspiracy to kill Yeonwoo, and that his own sister was involved (she did it for selfish reasons; her father refuses to let her marry Yeom (for political reasons - he wants Yeom to gain power and stand by Hwon's side and not be demoted to simply the princess's husband) and she was promised by the Queen Dowager that she would be able to marry Yeom, and she did). While Yeom forgives her - by this time, he loves her - Hwon does not. So Hwon demotes her to slave status (!) and sends her away as punishment. He later forgives her and allows her to return home to Yeom because they have a (precocious) child together.

Hwon takes Yeonwoo/Wol as his queen, and this is possible because Bokyeong commits suicide. Now, in the novel, she is not the evil witch that the drama is making her out to be. She's actually more a casualty in the war caused by her great aunt (Royal Queen Dowager by then) and her father (Lord Yun). When she discovers her father's plot to overthrow Hwon, she commits suicide to atone.

As for Seol... unfortunately (and this is the part of the novel where I cried buckets), she does not love Woon. She loves Yeom through and through. And when there is an assassination attempt on Yeom's life, she stands between him and the assassins and fights until her death. She saves his life. And before she fights, she asks Princess Minhwa (this is before Minhwa is sent off by her brother) and her lady-in-waiting to clean up her body in case she dies so that Yeom will not see her. So Yeom doesn't find out until much, much later that she died to save his life.

Do you know why Yang Mung died?

My memory is a little fuzzy, but from what I remember, someone attacks him with a sword or something, and he doesn't parry the attack. He just lets the guy stab him. Something to that effect. And Hwon is there to watch the whole thing. There's a "NOOOOO" moment, but I think everyone was too far to do anything.

As for Minhwa, I think the novel softens the blow a little by making her responsible a little indirectly (something like, she carries the talisman but doesn't plant it herself, or something to that effect). I don't remember if she knew she would be killing Yeonwoo, but the reason a lot of readers hated her is because she doesn't have any conflicted, guilty moments until Hwon finds out. Then she throws herself at his feet, saying, "I'm pregnant!" I hated her then, too. Hopefully, it will be better done in the drama.

Edit:: I just realized you asked why he died and not how. Well, basically, Hwon realizes that there's a conspiracy going on while he's king - Lord Yun and a few others are planning to overthrow him and replace him with YM. At this point, the Royal Queen Dowager has already left the palace in order to escape punishment in the future for her crime against Yeonwoo. So Hwon sends a secret letter to YM, ordering him to provide evidence of who is part of the conspiracy. So YM puts on an act, pretending that he intends to overthrow Hwon, and he gets everyone to sign a book of allegiance. Then on the day that they're storming the castle, he basically rides ahead of the traitors and hands the book to Hwon. Then he fights on Hwon's side. I think he dies near the end of the fight.

So as to why he died, perhaps he felt that he had nothing to live for? I am not sure. I always thought of his death as suicide

per la nostra coppia di innamorati....Sì.