The Arcturus Forest (アルクトゥルスの森 Arukuto~urusu no mori ) (also known 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 filled with old, corroded trees and brambles. 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 ley line portal created by the Shiny Rod's arrow allowed Akko, Sucy, and Lotte to escape. After Akko recites the final word and fuses the Shiny Rod with the Grand Triskellion, Arcturus Forest becomes a lush forest, blooming with multiple flowers, vibrant green trees, along with a rainbow that coats the sky.
The Arcturus Forest is a very old, sacred forest, having existed for centuries. In the 7th century, the Nine Olde Witches created a seal in the middle of the Arcturus Forest in which they sealed the Grand Triskellion and placed a magic circle to protect it and prevent others from approaching easily.
Later, the young witches Chariot du Nord and Croix Meridies went to the forest having learned about the Grand Triskellion. There, Chariot was chosen as the carrier of the Claiomh Solais. While she was searching for the sixth Word of Arcturus, Chariot tried to climb the Wagandea, only to fall and be saved by Croix.
As they were heading to Luna Nova Magical Academy through the Ley Lines, the students Akko, Lotte and Sucy accidentally fall into the Arcturus Forest due to the ley line stream distortion caused by Akko's pickled plums. Looking for a way out, they end up being attacked by a Mandrake and a Cockatrice. However, the Shiny Rod appears from the ground before Akko, choosing her as his new owner. When Professor Ursula arrives, she instructs Akko to use the first Word of Arcturus, leading her to revive it and invoke the Shiny Arc, opening a portal to Luna Nova.
An old dragon named Fafnir steals the Sorcerer's Stone and takes it to his house in the Rastavan Ruins — a kind of ancient magical ruins located in the outskirts of the Arcturus Forest. Akko and her friends travel to the border of Arcturus in order to reach Rastavan Ruins and retrieve the Sorcerer's Stone.
Akko discovers that Chariot revived the Sixth Word of Arcturus when she climbed Wagandea — a legendary, endless tree located in Arcturus. Akko is tempted to do the same act as her, but Ursula refuses because of the danger of Wagandea's poisonous magic-stealing pollen. However, Croix takes Akko there and tricks her into climbing the Wagandea tree, knowing that the poisonous pollen will soon spread, which will cause Akko to lose all of her magical abilities. She battles Ursula when she arrives in Arcturus and tries to defeat her. However, Croix stops due to Ursula's fall and possible death caused by her Magitronic devices. After a quick recovery, Ursula rushes to rescue Akko, who has been affected by the poisonous pollen and falls down from the tree. Croix had told her Magitronic devices to not go after Ursula due to the fact she knew she was too headstrong. Ursula requests the tree to not take away Akko's magical power and future, making the tree keep Akko's magical power, at the cost of Ursula's broom flying ability.
After Croix manages to accumulate enough emotional energy to activate the Noir Rod, she transforms the rod into a jet and sets off for Arcturus Forest, leading Ursula to rehash and jump into the jet as it is about to take off. Akko and her friends, who are returning to Luna Nova Magical Academy using the ley line stream, fall into the forest when the ley line stream darkens and disappears.
As Akko and her friends make it to the center of Arcturus, Croix manages to break the seal of the Grand Triskellion, but is greatly disappointed with what she ends up with. As it gathers more emotional energy, the powered-up Noir Rod is no longer within control and tries to kill Croix, but is saved by Chariot, who is then consumed by the Noir Rod monster. After she and Diana save Chariot, Akko has a heart-to-heart conversation with her, and she thanks everyone for helping her grow up. As Akko and Chariot hug, the Shiny Rod starts glowing, indicating that the revival of the Seventh Word is close. As Akko puts feelings into her words, she manages to revive the Seventh Word and uses the power of the Grand Triskellion to turn the Arcturus Forest into a beautiful and vibrant forest.
When everyone discovers that the Noir Rod has possessed a missile, Akko and her friends team up to stop it by counter-attacking it with magic. Chariot and Croix watch them from the tower of the forest and they get everyone in the world to know what's happening and to give them their support. When Akko and Diana succeed in destroying the missile and releasing the Grand Triskellion in the world, Chariot and Croix observe everything under Professor Woodward's gaze.