While there is no episode officially titled "Goodbye Fergus," that phrase refers to the character (whose human name was Fergus). If you are looking for a key moment where a character says "Goodbye, Fergus," it occurs later in Season 12, Episode 13, "Family Feud," when his son, Gavin MacLeod, chooses to return to his own time to die, effectively saying a permanent farewell to his father. [S12E04] " American Nightmare " Summary

The fourth episode of Supernatural Season 12 is actually titled .

Dean struggles with his mother Mary’s decision to leave and take some space for herself. "Supernatural" American Nightmare (TV Episode 2016) - IMDb

In this episode, Sam and Dean investigate a series of mysterious deaths involving in Iowa.

The brothers encounter the Petersons, a devout religious family living off the grid.

They discover the family is hiding their daughter, Magda , in the basement. Magda has psychic abilities and her mother, Gail, believes she is possessed by the devil and forces her to self-flagellate.

Sam helps Magda realize she isn't evil. However, after she is sent away for her safety, she is assassinated by Mr. Ketch of the British Men of Letters.