The Great Mothers' Fortress, the power center for the Dathmiri witches known as the Great Mothers, was constructed within a mountain located on the extragalactic planet of Peridea. This fortress contained an underground temple and a prominent spire crowned by a circular henge. Furthermore, it functioned as the operational headquarters for Grand Admiral Mitth'raw'nuruodo's departure from Peridea. To impede the pursuing Jedi, Thrawn ultimately destroyed it.
The Great Mothers' Fortress was the dwelling place of the witches Klothow, Aktropaw, and Lakesis, who are known as the Great Mothers of Peridea. Later, after being banished from his home galaxy during the events of the Liberation of Lothal, Grand Admiral Mitth'raw'nuruodo discovered the fortress and awakened the Great Mothers residing within. He then formed an alliance with them, using their magick to repair his Star Destroyer, the Chimaera, and establish contact with one of his agents in the primary galaxy. Thrawn's reciprocal agreement involved transporting the Mothers and their cargo with him.
Following a journey through extragalactic hyperspace, Morgan Elsbeth, Thrawn's agent, arrived above Peridea on the Eye of Sion, with the intention of transporting Thrawn and the Great Mothers to Dathomir. She and her mercenary forces reached the fortress and received a greeting from the Mothers. Shortly thereafter, the Chimaera docked above the fortress and initiated the transfer of cargo onboard. Thrawn, operating from the fortress, masterminded the scheme to use Sabine Wren in order to locate and assassinate Ezra Bridger, as well as the interception of Ahsoka Tano. Although the Jedi survived these attempts, the cargo transfer was successfully completed, and preparations for departure commenced. Tano, Wren, and Bridger infiltrated the fortress in an attempt to prevent their escape, but Morgan Elsbeth remained behind to stall the Jedi, providing Thrawn and the Mothers with enough time to evacuate the fortress.
Thrawn gave the order for the Chimaera's turbolasers to bombard the fortress, in an attempt to slow the Jedi's progress, which visibly angered the Great Mothers. Tano and Wren narrowly managed to escape, but the reanimated Night Troopers were obliterated when the structure collapsed.