Mystic Isle is a location from the 1985 animated series She-Ra: Princess of Power.
She-Ra: Princess of Power
Mystic Isle was an island located on Etheria where the First Ones once stored their greatest treasures such as paintings, arts and statues. Once done, they sunk the island beneath the ocean where it was protected and only rise from the waters at a specified point in the future. Legends would speak of the Isle and its treasure though the contents were not known.
During the Evil Horde's occupation of Etheria, Shadow Weaver learnt that the rising of Mystic Isle was coming to pass. She would inform Hordak of the time and he would seek to plunder it of weapons that he intended to use to destroy the Great Rebellion. Light Hope would call upon She-Ra and urge her to protect Mystic Isle from Hordak who was travelling to the site on a Horde Dreadnought. Upon reaching the island, he was disgusted to learn it contained no weapons but art and he sought to destroy it but She-Ra managed to thwart his plans.