The Arcturus Forest (アルクトゥルスの森 Arukuto~urusu no mori ) (also know as the Forbidden Forest) is a forest of ancient legend inhabited by many dangerous magical creatures and plants. It is there where the Grand Triskellion was sealed.
The Arcturus Forest is a huge and leafy forest covered with old and corroded trees. It is a sacred land which ordinary witches are unable to enter, protected by a magic circle that according to legend prevents even witches from leaving the forest once they've entered it. However, a leyline portal created by the Shiny Rod's arrow allows Akko, Sucy, and Lotte to escape.