The Forbidden Mountain is a range of mountains with a ruined castle at its peak that serves as Maleficent's home. The surrounding sky is generally pale green in color and the entire area has a dark atmosphere. The Forbidden Mountain is thought to be one of the gloomiest and darkest places to live.
In addition to the foreboding surroundings, various additional deterrents like eerie quietness, menacing statues, crumbling and decaying architecture, guards patrolling, and the power of Maleficent herself would deter any would-be intruder. This made the Good Fairies doubly fearful and cautious when sneaking in to rescue Prince Phillip.
Maleficent lives in an eerie castle within the Forbidden Mountain throughout her life, ruling over her own subjects, harnessing her dark magic, and scheming against Princess Aurora and King Stefan’s kingdom .
When her goblin soldiers report back to her about Aurora (they were searching for a baby) with no such luck, Maleficent becomes frustrated and zaps them with lightning before ordering them to leave her in her throne room. She takes matters into their own hands and sends Diablo to find Aurora and complete her dreaded curse of permanent sleep upon the unsuspecting princess on her 16th birthday.
After kidnapping Prince Phillip, Maleficent locks him in her dungeon in order to prevent him from rescuing Princess Aurora and the rest of the kingdom residents from their sleeping curse. The Three Good Fairies however, manage to sneak inside the castle and rescue him. They give him the Sword of Truth and the Shield of Virtue as he will encounter many dangers ahead.
When Maleficent finds out that Prince Phillip has escaped, she does everything she can to prevent him from reaching King Stefan's Castle. In the end, Maleficent, while still in her dragon form, is fatally stabbed in the heart by Phillip’s Sword of Truth, which allows him to save everyone in the kingdom, including Aurora, King Stefan, Queen Leah, and King Hubert. It is unknown what becomes of the Forbidden Mountain afterward.
Despite Maleficent's main domain being the Moors, the Forbidden Mountains make a cameo in the live action adaptation as the ruined castle that Maleficent takes refuge in after Stefan steals her wings.
It first appears in the second episode of Once Upon a Time, known as the Forbidden Fortress. A part of the Enchanted Forest, it is within the borders of King Stefan and Queen Briar Rose's Kingdom and was home to Maleficent. Regina would often make many visits here to seek the counsel and advice of Maleficent.
The Forbidden Fortress make's an appearance in Once Upon a Time in Wonderland episode "Forget Me Not".
It was were the Looking-Glass of Wonderland was held. Robin Hood, Will Scarlet and the Merry Men all raided the fortress in search for gold. Will himself had planned the raid as a distraction so he may search for the Looking-Glass and the ability to cross dimensions so he and his Anastasia could escape the Enchanted Forest for a new realm, Wonderland.