X is for the X-th dimension
The X-th dimension is a postulated extradimensional realm where, according to some pseudo-scholars and doomsday cults, a massive army of alien creatures is being raised to carry out an invasion of the known planes. Proponents of this theory believe only a finite numbers of alternate dimensions exist, the X-th one being as of yet unexplored or unreachable by commonly practiced aetherial magick. This has however not stopped various self-professed prophets and other supernaturally or narcotically inspired soothsayers to make unverifiable claims and write esoteric treatises concerning the nature and particulars of the hypothetical X-th dimension.
Of considerably greater value and importance on said topic are the writings of the renowned sage Paronax, who theoreticized on the subject in his Catalogica Planorum, the definitive work on aetheromancy and portal summoning. In his magickal calculus Paronax assumed the existence of an X-th dimension to compensate for the unexpected and inexplicable discrepancies that his otherwise sound arithmetic produced. He deduced that if one were to use a sufficiently powerful source of energy, one could penetrate the border between this world and the X-th dimension. Such a feat would require extraordinary amounts of power however; something only a few known artifacts would prove capable of, and the risk of irreparable harm to it would be great. The costs of conducting a practical experiment thus were considered too high, and decades after Paronax wrote his Catalogica his theory remains untried.