Memeovore

A Memeovore, otherwise known as the Fendahl Predator, was a creature that devoured meaning and concepts. (PROSE: The Taking of Planet 5)

Specifically, the Memeovore fed on the energy released during the collapse of probabilities caused by observation. This caused the normal-scale cosmos to break down and disrupt the brains of the observers, essentially 'feeding' on the channels of comprehension that existed between every form of life in the universe. The Memeovore was known to target the most complex system in any environment, such as the brain of an observer, but also potentially a realm constructed out of thought, such as Mictlan. (PROSE: The Taking of Planet 5)

History
During the War in Heaven, the Great Houses considered multiple ways to create a Memeovore as a weapon against the Enemy. (PROSE: The Book of the War) Unbeknownst to the Houses, the last species on Planet 5 was enabled by a time loop at accelerated speeds to evolve into a Memeovore to kill and feed on the Fendahl.

When a faction of Time Lords used a herd of TARDISes to retrieve the Fendahl for use against the Enemy, the Memeovore escaped into the universe, devouring meaning and concepts across space-time. It physically appeared as a living void, with those who looked at it seeing an endless procession of grotesque images as their minds struggled to understand its incomprehensible form. Under the Memeovore's universe-spanning influence, it consumes the concept of "family" on the worlds of the Nepotism of Vaal, a human Earth colony in the future loses the ability to comprehend the concept of “circles”, the alien Delphon species lost their ability to use language and communicate, and reality slowly began to collapse from having a lack of meaning. The Memeovore later locates Mictlan, the home of the conceptual entities known as the Celetis, and a realm of pure concept that would be a particularly rich feast.

As the Memeovore moved to devour Mictlan and destroy the Celestis, the Eighth Doctor was able to convince the remaining War TARDISes to help him defeat it by forming themselves into trans-dimensional blades, essentially cutting Mictlan and the Memeovore out of the universe and expelling them into the outer void. It was later revealed by the Celestis agent One that the Memeovore was released for the purpose of preventing the Swimmers from being attracted to the universe. (PROSE: The Taking of Planet 5)

Adam Smith was described by Torchwood as a Memeovore. (AUDIO: Madam, I'm)

The Fifth Doctor once came upon a Memeovore that escaped containment on the spaceship Pride of Sheboygan. The Memeovore had eaten the memories of the ship`s surviving crew before being taken by the Doctor to the planet Soloto where it can live in peace. (PROSE: Piecemeal)