The Forbidden Marriage S01 E06 Episode 6

The appearance of Ja Yeon's ghost stirs up the palace. However, Shin Won and So Rang investigate thoroughly to prove to Heon that she is not real. Eventually, they find out that someone has stolen the late Princess' court dress. The ghost is a person who just resembles Ja Yeon and is part of a scheme made by someone with a high rank within the palace.