He-Man and the Masters of the Universe
The Dark Dimension is the dimension that Daimar the Demon came from.
She-Ra: Princess of Power
The two mortal enemies were forced to work together if they wanted to find a way out provided by the Dark One, the master of the Dark Dimension.
When Hordak tried to betray She-Ra and leave without her, the Dark One trapped him in a stasis field and told She-Ra she was free to go home. But the Princess of Power proved her worth by refusing to leave without Hordak.