It is a dark realm. Some who have come from other worlds and visited it called it Hell, others, a Mirror Universe, to those who live there it's simply home. The Mirror Universe is morality reversed. Evil rules here with an iron fist. It's a world where The Terran Empire was reformed and allies themselves with the Klingon/Cardassian Alliance, The Crime Syndicate, and The Dark Jedi, and The Interstellar Empire, The Dark Magical Underworld, and Fallen Angels rule. They eliminated The Justice League, The Dominion, Heavenly Demons.
This world is commanded by Emperor James T Kirk, Ultraman, Intendant Kira, Empress Selene, Emperor John Sheridan, Superwoman, Emperor Anakin Skywalker, and Owlman. They Split factions of a universe and have their own respective Empires. They tend to step on eachother's toes from time to time but they somehow maintain their alliance due to the fact that it's good for business.
But then beings from other realms, good or evil, entered this dark universe. How they arrived is unknown. Some actually arrive because they want to escape. Others come to simply explore universes. Some come to conquer. And some come out of random chaotic chance. From this, new rivalries arrise and new alliances forged.
This world is corrupted, twisted, evil, bleak. Full of mystical energies, starships, mutants, and other super-powered beings. In this dark world, heroes are rare and are outcasts. They still fight what they know is evil, which is sadly for them, the world itself.