They were a living absence which insinuates itself into subatomic quantum interactions thus collapsing the alternate probabilities by the act of its observation and feeds upon the energies that are released. This makes them Memeovores, creatures that devoured concept itself and were capable of encompassing all of time and space.
A faction of Time Lords used a herd of TARDISes to break the time barrier around the Fifth Planet in order to free the Fendahl for use against the Enemy. Unbeknownst to them, the Fendahl no longer existed and they released the Fendahl Predator instead. (PROSE: The Taking of Planet 5)