Mortis was an ethereal realm within the Force that was whispered over in legends and stories passed down through the millennia by Jedi, Sith, and the average spacer alike. It was believed to have existed outside any star system, and potentially the galaxy or even the universe. Some believed that the origins of the Force came from Mortis.
Mortis after nightfall
Mortis was a realm of the Force that contained dreamlike environments; it could not be mapped, and even its landmarks appeared in different locations frequently. The daytime was a time of peace during which blossomed lush fields, green mountains, and stunning vistas. Dense forests, molten caverns, and floating mountains above the surface were among some of the different biospheres. The light had a healing quality, although it came from no star. When night fell, the plants withered and died, the creatures vanished, and the few stars that shone in the black sky were unidentifiable. Scalding rains swept the terrain, making survival without shelter nearly impossible.
Entrance to the mysterious planet was never voluntary, as Mortis drew in its visitors inexplicably. It manifested itself in different ways; Mortis was reported as having appeared as a massive monolith floating in space; other times individuals walked through a door or landed there without discernible reason. Time within Mortis had no meaning; what seemed like days or weeks within the realm, passed within seconds without, a mysterious quality it shared with such locales as Ahch-To and Dagobah.
The realm of Mortis was inhabited by only three intelligent beings, Force wielders locked in an eternal struggle for dominance over one another: the Son, who represented the destructive and deadly aspects of the realm and was aligned with the dark side of the Force; the Daughter, who represented peace and creation on Mortis and was aligned with the light side of the Force; and the Father, who maintained the balance of the Force between the two. Mortis was therefore a fulcrum for the galaxy and an extremely powerful conduit through which the Force flowed.
Mortis during the day
During the Clone Wars, the Father drew Jedi Knight Anakin Skywalker, Jedi Master Obi-Wan Kenobi, and Jedi Padawan Ahsoka Tano into the realm from the Chrelythiumn system. The Father realized that Skywalker was the prophesized Chosen One, and that he could maintain the balance of the Force between the two children once the Father died. This led to a struggle between the Force wielders and the three Jedi, culminating in the deaths of all three wielders and the restoration of balance on Mortis.
The ethereal realm shared its name with the Phases of Mortis, a solution to the Unsolved Theorem of Master Thorpe that was included in the sacred Jedi texts.
Mortis first appeared in the episodes "Overlords," "Altar of Mortis," and Ghosts of Mortis" in the third season of the animated television series Star Wars: The Clone Wars. It was later mentioned in the Star Wars Rebels episodes "Wolves and a Door" and "A World Between Worlds," with the Mortis gods also pertaining to the mysterious and mystical plane known as the World Between Worlds.