Limbo parasite

The Yeerks' seaweed cousins.

Limbo parasites are lifeforms native to Limbo. They appear as green blobs that are telepathically linked to one another and feed on emotion, in particular aggression. They do so by attaching to their preys' heads to both scan their minds and generate dreams that get the optimum violent reaction out of them. If given enough time, the parasites are even capable of making the illusions real.

Fiction

Marvel The Transformers comics

The Limbo parasites appeared exclusively in the UK portion of the Marvel Comics continuity.

Optimus Prime, Ratchet, Prowl, Shockwave, Thundercracker, and Frenzy were all transported to Limbo when a bunch of future Transformers time-jumped to 1986. They were immediately caught in an illusion of a war between the Cloran and a technorganic apelike species that suspiciously resembled the war between the Autobots and the Decepticons. Optimus Prime realized this and by refusing to let himself be provoked broke the parasites' control. Frenzy wished to tear the parasites apart to put a stop to them, but Shockwave forbade him to do that, as it was likely that any act of aggression would start the whole thing over again. Optimus Prime noticed that wounds he had obtained during the dream were still present on him, causing the Autobots to conclude that prolonged exposure to the parasites' influence would be lethal. Distant Thunder!

During the Time Wars, many more Transformers were transported to Limbo and all fell prey to the parasites. The only one who was left alone was Optimus Prime, since he had technically become immune after his first encounter. Because he was not part of the illusions generated, he could do nothing to help his fellow Transformers but hope that they would soon be freed from Limbo. Time Wars One particular bad case was that of Skids, who had been stuck in Limbo since way before the Time Wars. His dreams had already began seeping into reality. When he too was freed after the Time Wars and returned to Earth, he brought along a slew of monsters bent on world domination. Survivors!

Animated cartoon

During the Great War, Skids was trapped in a time loop where he got consumed by limbo parasites. The AllSpark Almanac II

Ask Vector Prime

The Time Warriors and Tic Toc tricked Time Limit into entering Limbo. He ended up being a tasty meal for the parasites and was rendered inert. Ask Vector Prime, 2015/09/21