Non-worlds were locations or areas of reality which had a causal relationship to the Spiral Politic, but no physical existence in the conventional universe. As such, they were a source of great annoyance to the Great Houses during the War in Heaven.
The three most notable Non-Worlds involved in this Time War were Mictlan, a realm of pure ideas created by the Celestis to serve as their lair; the Eleven-Day Empire, a time-looped shadow-version of London serving as the base of operation of Faction Paradox; and the City of the Saved, (PROSE: The Book of the War) the realm of the timeship Compassion (PROSE: Of the City of the Saved...) which stood beyond the end of the universe. Additionally, the Thirteen-Day Republic was created by a breakaway arm of Faction Paradox in the eighth year of the War in Heaven in direct imitation of the Eleven-Day Empire. (PROSE: The Book of the War)
The Mandarin had been tracking Aboo-Fenran in the Arabian plains, but wasn't able to reach him before Fenran was "hurled" into a non-world, (PROSE: Games [+]Warwick Gray, Brief Encounters (Marvel Comics UK, 1992). Page 22.) the Shadow Dimensions, which the Seventh Doctor sent him to after tricking him into growing progressively weak enough to be contained in a flask. (PROSE: The Curse of Fenric [+]Ian Briggs, adapted from The Curse of Fenric (Ian Briggs), Target novelisations (Target Books, 1990).)
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