It is an underground world unknown to humans. It was discovered when Emily Briggs, Looker, was taken to Abyssia by the Abyssians so Emily could take her place as Queen and end the fight between Mardo and Tamara. The Outsiders were made aware of her capture and went to save her. Emily was turned into a metahuman when Haley's Comet passed by in the right setting. She then ended Tamara and Mardo's lives, or so they thought. Looker and the Outsiders left the Abyssians to be ruled by two nice people who helped them.
The Outsiders Return
Roderick and a controlled Looker, Who was now a Vampire, took over Abyssia and used the Abyssians for a blood bank. The Outsiders once again stepped in to try and stop this. Tamara also made an attempt to stop them, as she wasn't killed in the previous encounter with the Outsiders. Abyssia was once again left to peace and to rebuild it's population after the Outsiders defeat Roderick and make Looker become part of the team.