Memeovore

The Memeovore was a creature that could feed on meaning. Specifically, it fed on the energy released during the collapse of probabilities caused by the observation. (PROSE: The Taking of Planet 5) This caused the Memeovore to target the most complex system in any environment, such as the brain of an observer or the realm of Mictlan.

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 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. It devoured meaning across the cosmos before homing in on Mictlan, a manifestation of pure concept that would be a particularly rich feast.

As the Memeovore moved to devour Mictlan, 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 out of the universe and expelling it into the outer void as the Memeovore was feeding on it. (PROSE: The Taking of Planet 5)