Limbo is a holding dimension used during the process of mass substitution when time travelling. It is a black featureless void inhabited by green emotion parasites. The creatures attach themselves to any being that arrives in limbo and uses mental manipulation to stimulate the violent emotions on which they feed. Those ‘infected’ by the parasites experience their dreams as though it was a reality. Given enough time and sufficient violent emotion the creatures can create an actual reality from the dreams.
Marvel UK comics continuity
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 are drawn into the battle until Optimus Prime sees the similarity between this war and the Autobot/Decepticon conflict. He orders the Autobots to stop fighting just in time as the dream is being made a reality. He suffers a wound when Zenag attacks him but as the fighting stops the dream fades and they all find themselves in the void with the parasites attached to their heads. Once the time travellers return to their own time all six were released from Limbo.
When Galvatron later returned he dispatched Skids to Limbo. Skids remained there for several years until Galvatron was destroyed by a time rift. Due to 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. They were eventually destroyed by the 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.