Mistress Aloway was an unseen villainess mentioned in "Family Feud," episode 12.13 of Supernatural (airdate February 23, 2017).
In 17th century Scotland, Mistress Aloway was a schoolteacher for many children, among them being Gavin MacLeod, the son of Fergus MacLeod (Crowley's vessel). She later became a crewman for The Star, Gavin's ship, which set sail for America in 1723. Originally, Gavin was part of the expedition, along with his fiancee, Fiona Duncan; however, Gavin was transported to 2014 by Abaddon, leaving Fiona to enter the ship as a stowaway while waiting for Gavin. After the ship's crew discovered Fiona, they ended up continuously raping her, with Fiona looking to Mistress Aloway as a means of protection.
Instead of protecting Fiona, Mistress Aloway turned heel and encouraged the crew's actions, allowing the sexual assaults to continue. The ship sank during a storm and killed everyone on board, including Fiona and her villainous teacher, and it was Mistress Aloway's heel turn and callousness that led to Fiona emerging as an evil and vengeful spirit in the episode's events, which saw her kill teachers for revenge over the actions of the evil Mistress Aloway. The episode ended with Gavin returning to the timeline and the ship, as well as his original ill fate, though he protected Fiona up until the ship's crash.