“The Taking of Planet 5”
The Memeovore was a creature that evolved from the last life form on Planet 5 other than the Fendahl. The Memeovore evolved specifically to kill the Fendahl.
The Memeovore was capable of devouring meaning itself, ‘feeding’ on the subatomic interaction of events caused by observing events as they took place, draining the energy that could result from the multiple outcomes of any event while retaining only its own observation of what was taking place. This caused the normal-scale cosmos to break down and disrupting the brains of the observers- the most complex structures present- essentially ‘feeding’ on the channels of comprehension that existed between every form of life in the universe.
After devouring the last of the Fendahl, the creature was released onto the rest of the universe when the Time Lords of the War broke the time loop around Planet 5. As a result, the Memeovore was released into the universe, devouring meaning across the universe before homing in on Mictlan, the home of the Celestis and a manifestation of pure concept that served as its richest feast. As the Memeovore moved to devour Mictlan, The Doctor was able to appeal to the remaining War TARDISes to help him defeat the Memeovore by forming themselves into trans-dimensional blades, essentially cutting Mictlan out of the universe and expelling it into the outer voids.
This was part of a plan conceived by One, a renegade Celestis agent. One had become aware of the Swimmers, powerful beings larger than universes who existed in the void between universes, crushing universes as they swam, and had learned that Mictlan might attract the Swimmers to this universe. To this end, he had developed the plan to destroy Mictlan, preventing this universe from attracting the attention of the Swimmers and leaving him and his mentor the last of their people. (The Taking of Planet 5)