The Undercroft was a secret hall within Hogwarts Castle accessible through a hidden passage on the ground floor of the Hogwarts Turris Magnus, as well as via an ancient magic passageway to Isidora Morganach's house at Feldcroft.
History[]
Knowledge of the existence of this room passed through the House of Gaunt for generations. It was shown to Sebastian Sallow by Ominis Gaunt, the latter of whom named it the Undercroft. Before Anne Sallow left Hogwarts, she, Sebastian, and Ominis would often play Gobstones in this room.
In his fifth year, Sebastian shows you the Undercroft, teaching you the Blasting Curse. This irritated Ominis, who felt his secret had been betrayed, though as the school year advanced, you all become close friends. Sebastian and you, meanwhile, discovered an ancient magic link between the Undercroft and Isidora Morganach's house in Feldcroft. After travelling through, a triptych opened which depicted a torn scene of the Highlands, tempting Sebastian to explore the area and learn more about Morganach. When both of you finished the triptych, a Pensieve appeared which showed Morganach's life in the period.
In the later part of the school year, following the murder of Solomon Sallow by Sebastian, Ominis Gaunt brings you to the Undercroft to ask if you should turn Sebastian in.