The Leibniz Universe was discovered by Valkyrie Cain when Silas Nadir shunted her there in Kingdom of the Wicked. Valkyrie dislikes the name and instead wishes it to be called Dimension X.
When Nadir wakes up from his coma (when he is shunting the prison through dimensions), he reaches for Valkryie’s arm and uses his magic in a way that means she will shunt into a different dimension. He has no recollection of doing this, and it is later revealed that this was caused by Armageddion controlling him.
In the Leibniz Universe, Mevolent and the sorcerers rule the mortals, who are almost like slaves. Sense-Wardens and Redhoods (similar to Cleavers, but wearing red) roam the streets—Sense-Wardens read the minds of the mortals and they are taken away if they merely think anti-Mevolent thoughts. Several cities exist, including Dublin-Within-The-Wall, Tahil na Sin and Tahil na Kurge. Initially, Valkryie thinks that this universe diverged from hers due to Mevolent’s survival, but it is later theorized that it was rather Skulduggery not taking off Lord Vile’s armour that changed things.
The Resistance is a group of Sorcerers who are still against Mevolent, led first by China Sorrows, then by Nefarian Serpine, and then by Eachan Meritorious.
It is also infested with the Draugr, which Lord Vile created to wipe out the survivors of the universe, as he felt that he needed to kill everyone to end human suffering. This is also one of the reasons that Mevolent wanted to enter our dimension.