Shin Min Ah, who played as Gumiho in her previous drama. It looks like she enjoyed being a "ghost" character.
Here's some synopsis, taken from several source:
The drama is based on famous folklore, as all ghost stories are: During the Joseon era in the city of Miryang, Arang was the pure, beautiful, goodhearted daughter of a magistrate. She grew up without a mother, and was raised by a wicked caretaker who conspired to have her be raped and ruined by a servant. He attacked and she resisted, so he stabbed and killed her, and left her body to rot in the woods.
Her father the magistrate just believed that she dishonorably eloped with a man and so resigned his position swathed in shame. Thus the legend goes that every time a new magistrate comes to Miryang to fill the position, Arang’s vengeful ghost shows up to tell him her story and he flees in terror.
But one day a new magistrate comes to town — a young man by the name of Lee Eun-oh. Arang appears to him like every other magistrate before him, but this man doesn’t flee, and instead sympathizes with Arang, and promises to find her killer and avenge her death.