Halfmoon is a location from the 1985 animated series She-Ra: Princess of Power.
She-Ra: Princess of Power
Halfmoon is an underground kingdom inhabited by the Magicats.
When the Evil Horde first began it's occupation of Etheria, the entire race of Magicats fled below ground and stayed there, living in secret. Only the Magicat Queen ventured outside to search for members of her species that had been captured, but soon disappeared. The remaining Magicats kept hoping that one day their Queen would return.
The underground kingdom of Halfmoon is difficult to find even in Subtheria. It is surrounded by walls, outside of which lies an underground maze.
Since the Magicats were said to have fled to the underground Kingdom, it must have already existed and been known to them even before the Horde occupation.