Forbidden Mountain is thought to be one of the gloomiest and darkest places to live. Maleficent lived in a creepy castle within the Forbidden Mountain throughout her life ruling over her own subjects, harnessing her dark magic and always scheming such foul plans.
When her goblin soldiers reported back to her about Princess Aurora (only they were searching for a baby) with no such luck, she became frustrated and ordered them to leave her in her throne room. She and her pet Diablo take matters into their own hands and left Forbidden as they went to find Aurora and complete her dreaded curse of pernament sleep upon the unusupecting princess on her 16th birthday.
It's the place where Maleficent kidnapped Prince Phillip and locked him in her dungeon, in order to prevent him to rescue Princess Aurora and break the spell. 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 by reaching the castle.
Forbidden Mountain appears in the Enchanted Dominion level of Kingdom Hearts: Birth by Sleep.