X-Dimension
The X-Dimension is a universe of flame and radiation. Despite the inhospitable conditions, it is home to the X-Dimension Microns.
Fiction
The AllSpark Almanac
Aero-Bot led an ill-fated five-person expedition from Yayayarst 201.11 Beta into the X-Dimension. Transformers Animated: The AllSpark Almanac II
TransTech
Facing expulsion from Axiom Nexus and his home reality destroyed, Ghoulon volunteered to join an expedition to the X-Dimension. He was not heard from since. Andromeda - Axiom Nexus News Reporter
Ask Vector Prime
The Planet X of Primax 994.0 Gamma, under command of the Cybertronian Empire, arrived in this dimension and attempted to conquer it. The inhabitants, however, fought back, and the war ended up scorching the entire reality, leaving very little energy or life. Planet X then traveled to another reality, their method of which shattering the dimension entirely and leaving it in its current, inhospitable state.Ask Vector Prime
Games
Transformers Universe
Flatline's "Dimension Splitter" ability let him phase into the X-Dimension, rendering him intangible and healing his wounds. The ability also generated an aura that repaired his allies. Flatline's Transformers Universe profile
Legacy
Aside from The Allspark Almanac, The X-Dimension has never actually appeared in any fiction. According to the Micron Densetsu Year Book 2003, it was originally intended to be the subject of a mini-comic that starred the thirty Japanese-exclusive "X-Dimension Mini-Cons" released by Takara, telling the story of this parallel universe from the little bots' perspective. This comic never materialized, although in 2003/4, a 13-part mini-comic "Linkage" was produced as a pack-in for the Japanese Micron Legend (Armada) DVD-release, which retold the Armada conflict from the perspective of the Mini-Cons...but in the mainline cartoon continuity, rather than an alternate universe. It seems probable that the X-Dimension comic was dropped in favor of "Linkage" when Takara realized that the Armada cartoon was borderline incoherent a bit messy in places and needed someone to clean house more than they needed an alternate-universe retelling.
Beginning in 2009, several of the X-Dimension Mini-Con toys were repurposed to represent unrelated characters from the Shattered Glass "mirror universe". This was likely a deliberate homage to the X-Dimension toys representing characters from an alternate universe...but Shattered Glass universe itself is not the X-Dimension.
