The Rusty Key is used to enter The Sewers. It is obtained from Gunther in a cutscene the morning after 60 items have been donated to the Museum, where he will greet the player at their front door, thank them for their contributions to the museum, and give them the key. In Multiplayer, the host must speak with Gunther to get the key.
Its existence is first referenced by Jas and Vincent in a cutscene in Cindersap Forest. This cutscene triggers on or after Spring 11 of year 1 by entering the Forest between 9am and 4pm on a Monday, Thursday, or Sunday.