Story:
1000 years ago, there was a pair of Bulgasal, a monster without souls who cannot die unless both pairs are killed concurrently, living in the mountain and promising each other to always stay together. However, female Bulgasal developed feelings for human children because she couldn't have her own, compelling her to abandon the male Bulgasal couple (other-self) temporarily. As the children grow up together with the female Bulgasal and their human adopted mother, they lived happily even when they find out about the female Bulgasal true identity until they witness a murder happening.

Ok Eul Tae killed his younger brother because his father had lost all confidence in him as he was dying and was attempting to make his younger brother his heir. Ok Eul Tae then discovers the already grown-up human children and attempts to murder them, only to be stopped by female Bulgasal. He realizes she is a Bulgasal and puts the blame on her causing all sorts of misunderstandings to take place. Ok Eul Ta's father and his soldiers try to slay the female Bulgasal, but they are unsuccessful since she is immortal. They intended to decapitate her and imprison her until the male Bulgasal arrived and slaughtered everyone. This led to misunderstandings between the two Bulgasal pairs. The guy intended to protect her from harm, but she despises him for murdering and cursing the ones she holds dear. To stop him, she stabbed her own heart in an attempt to kill herself and her pair but she fails when Ok Eul Tae takes him away to gain similar powers.

The male curse Ok Eul Tae with a dark hole in his chest, and steal his soul in a promise to free him from the curse if he turns his reincarnated human self into Bulgasal. 600 years later, The male Bulgasal is born as a human called Hwal with the Bulgasal curse. As a result, his mother dies and his father abandons him, and he is later adopted by Dan Geuk, a general and Dan Sol's father (Ok Eul's father in the past). Hwal marries Dan Sol after a few years (one of the human child female Bulgalsal raised). Hwal had two children, one of whom was born blind (the other kid raised by Female Bulgasal) and the other of whom died prematurely due to his curse. Therefore, Hwal avoids growing close to his blind son in order to show him love and affection, as his father had done to him. He eventually regrets it. Hwal embarks on a mission to break his curse, slaying all monsters along the way until he finally meets the female Bulgasal who has survived the event that occur 1,000 years ago. Hwal does not kill her the right way because he remembers that she had saved him in the past when he was a kid. However, due to Ok Eul Tae killing everyone to convert Hwal back into a Bulgasal in order for him to keep his pledge and be free of his curse. Hwal believes it was the female Bulgasal who did it, so he sets out to find her to avenge his dead wife and children. Little did Hwal know that the female Bulgasal would steal his soul and turn him into a Bulgasal but she tells with that she will be born again as a human. So the cycle of revenge begins again.

Fast forward to the present time, Hwal meets Sang Min (a female Bulgasal who has reincarnated as a human) to seek revenge but discovers the existence of another Bulgasal (Ok Eul Tae). He also encounters other humans, including Shi Ho Min (his reincarnated wife), Nam Do Yoon (his reincarnated blind son), Detective Kwon (his reincarnated adoption father), Miss Lee (shaman), and others. Then, it's just the process of finding the truth of the past. While they are doing so, we learn that Ok Eul Tae has developed into a true monster who is willing to go to any length to stay immortal and relieve himself of his curse. WE also find out that he has joined hands with another human (monsters in a past life) to kill the female Bulgasal reincarnation so he can stay alive. When Hwal realizes Nam Dong is secretly assisting Ok Eul Tae, he warns him to leave before harming him. He later discovers that Nam Dong is his reincarnated son and that Ok Eul Tae is using him, so he saves him from being killed.

Despite the fact that Hwal has turned into a monster, we get to see a more human side of him. Even though he has avoided being close to Miss Lee because of her past life as a shaman who led others around him to harm him, their relationship is comparable to that of a father and a daughter. When Miss Lee is murdered, Hwal realizes Lee Hye Su is like his daughter and regrets not becoming close to her, just like with his blind son in the past. The cycle of abandonment and regrets continues. There is more to it than resentment and grudge, there is a deeper meaning behind those causes, such as family, love, and regrets. After the past is revealed (this is in the very end but there are a lot of things happening before it's revealed), Hwal makes the decision to assassinate Ok Eul Tea and himself. Many people, including Detective Kwon and Sang Min, die in the process. Then we flashed forward to the future (8 months, 6 years, 30 years, and 50 years) to find that the survivors, Shi Ho min and Nam Yoon, have their own big families. We also see Hwal and Sang Min reuniting in the future, 50 years later, in the old house they lived in with others while searching for the truth. ......my summary ending is rushed just like how this show ends. XD

The actors did a great job executing their roles, and the directors and music director did a fantastic job as well, but the script was rather weak. It started out great and had a lot of potential, but then it got too draggy out, and boom. Suddenly, we get a rushed ending. Overall, Brilliant performances, amazing cinematography, compelling OST, an amazing concept of Bulgasal (curse and reincarnations), and a great blend of genres.