I watched this drama after listening to the OST sang by SNSD Tiffany. I thought it was a beautiful song and someone made an English cover of it and I liked their version a lot also. So upon finding out that it was an OST, I decided to watch this drama. I didn't like it that much but I'll give you a basic summary of the drama.
This is a historical drama. The Kingdom of Nangnang is possessed by a mystical drum called Ja Myung Go. It will make a sound by itself when enemies invade the kingdom. In reality, the drum is actually embodied by the King's daughter Princess Ja Myung. The king had two wives and Princess Ja Myung and Princess Ra Hee were born on the same day and were half-sisters. It was believed that one princess will be the nation's savior while the other will destroy the nation. Not knowing which one was the savior and which one was the princess of destruction, it was said to kill both girls. However, using her family's powerful background, Princess Ra Hee's mother manages to convince people that her daughter is the savior princess and must be kept alive.
Ja Myung was labeled as the princess of destruction and managed to escape death as a baby and grew up among common people who worked at a circus. When she escaped death, a little boy was there with her the entire way and when they grew older he became her brother in a way. When Ja Myung learns of her true identity as a princess, she returns to her kingdom. She falls in love with Prince Hodong of the rival state of Goguryeo. However, they cannot be together. Princess Ra Hee also fell in love with Prince Ho Dong and was even willing to give up her nation for him by destroying the Ja Myung Go. Princess Ja Myung continued to fight to save her nation even if it meant killing the prince she loved. In the end, Ja Myung was the savior princess and Ra Hee was the princess of destruction. As for what happened throughout the story, you can check it out if you want to.