150 years before the events of Gravity Falls Nathaniel Northwest commissioned the Ghost of Northwest Manor along with numerous other townsfolk to build him the Northwest Manor. In return they would host a grand party every year in which everyone in Gravity Falls was welcome. However, after they completed the manor the Northwests locked out the townsfolk, and since then the party only welcomes the super-rich and elite.
In "Northwest Mansion Mystery," the Northwests invite numerous people of note, such as dukes and duchesses, to the gathering. However, it is interrupted with the appearance of the Ghost of Northwest Manor. The ghost, previously thought to have been disposed of by Dipper Pines begins to turn everyone into wood. Then when he is about to burn the manor Pacifica Northwest opens the town gates, letting everyone into the party. Everything turns back to normal and the ghost's spirit is freed.