Limbo

C'mon, Frenzy, what did that seaweed ever do to you?

Limbo is a dimension separate from normal space but seemingly still subject to the normal flow of time. It is a black, featureless void inhabited by green mental parasites that attach themselves to anyone who enters. The creatures induce a shared hallucination in their victims, a simulated reality filled with conflict meant to inspire violent emotions. As the parasites feed off of these emotions, the experience of the victims becomes stronger, to the point of eventual physical manifestation. Conversely, if victims refuse to play along with the illusion, the creatures' power over them will fade and can break entirely.

The only known means of entering Limbo is a side-effect of mass substitution-based time travel. When a time traveller employing that method arrives at their destination, an individual of similar mass in that time will be instantly transported to Limbo. When the traveler time-jumps away again, the displaced subject will likewise reappear in normal space. The Creation Matrix has also been used to escape Limbo, though this causes damage to the space-time continuum by thwarting the mass-substitution effect.

Fiction

Marvel The Transformers comics

There's also Jimmy Hoffa in here.

When Galvatron, Cyclonus and Scourge time jumped back to 1986, they displaced Optimus Prime, Ratchet and Prowl to Limbo. Later Kup, Hot Rod and Blurr displaced Shockwave, Thundercracker and Frenzy. Target: 2006

All six found themselves in the middle of a war between the peaceful cat-like Cloran and evil monkey-like invaders led by Zenag. At first they were drawn into the battle until Optimus Prime saw the similarity between this war and the Autobot/Decepticon conflict. He ordered the Autobots to stop fighting just in time, as the dream was being made a reality. He suffered a wound when Zenag attacked him, but as the fighting stopped the dream faded and they all found themselves in the void with the parasites attached to their heads. Distant Thunder! Once the time travellers returned to their own time, all six were released from Limbo.

When Galvatron later returned he dispatched Skids to Limbo. Fallen Angel Skids remained there for several years until Galvatron was destroyed by a time rift. Time Wars Due to the length of time he was there the parasites were able to make a number of horrific creatures a reality. These creatures were later returned to Earth along with Skids, where they were eventually destroyed by the Survivors. Survivors!

Before Galvatron was destroyed, Optimus Prime was once again dispatched to Limbo when Rodimus Prime travelled back in time to try and seal the time rift caused by Galvatron. Having been to Limbo before he was able to resist the mind parasites. Using the link to the Creation Matrix that both he and Rodimus Prime carried, he was able to communicate with the Autobots on Earth. He then later used the Matrix as a way of escaping from Limbo without Rodimus Prime having to return to his own time. This bypassed the mass substitution process causing the time rift to worsen. Time Wars

Masterpiece toy bios

The sparks of many deceased Starscreams from all across the multiverse gathered together in Limbo, sharing the stories of their deaths with one another and formulating a plan to restore themselves to life. The scientific skills of several of the Starscreams were used to create a kind of crystal that would enable their spirits to retake physical form in the real world—but predictably enough, all the Starscreams then began fighting for a bigger share of the crystals, in hopes of achieving a more certain result. The native creatures of Limbo scattered to avoid the chaos of the battle, which ultimately concluded with all the Starscreams escaping as one, and inadvertently being fused together into a giant "ghostly" Starscream. MP-3G Destron Air Commander / Starscream Ghost Ver.

Ask Vector Prime

The Time Warriors and Tic Toc tricked Time Limit into entering limbo. Once there, he was swarmed by the parasites. Ask Vector Prime, 2015/09/21

Notes

• In real life, the term "Limbo" originally referred to the Roman Catholic concept of an afterlife on the boundary of hell, where unbaptized souls still tainted by original sin but otherwise deemed undeserving of damnation would rest.
• The version from the Marvel comic is much more of a physical dimension; it had no connection to the afterlife until the Masterpiece Starscream story depicted it as the resting place of the deceased Starscream's spark.