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The Ravenous were a race of creatures from the "old times".
In their natural state, the Ravenous resembled clowns, with the Doctor speculating that they inspired the traditional image of the modern clown as part of a universal racial memory attempting to make them less scary.
After defeating the Kandyman's and the Eleven's scheme with the thought spider, the Eighth Doctor and his companions caught of a glimpse of faces reflected in glass, screaming, and Helen Sinclair saw a word in her mind: "Ravenous." Afterwords, the Eleven mused to himself that he knew what they fed on, and that all anybody could do was run. (AUDIO: Sweet Salvation)
The Eleven was later found by one of the Ravenous onboard a dying TARDIS which he had stolen. The Eleven was scared enough to call for the Doctor’s help escaping it. According to the old tales, the Ravenous ate temporal energy, with Time Lords thus being their most potent source of "food", while shorter-lived life forms such as humans would be little more than the equivalent of a packet of crisps to satisfy the Ravenous' appetite. After the Eleven stole the Doctor's TARDIS, the Doctor was able to escape the Ravenous by using the last of the dying TARDIS's power to make a random materialization, leaving the Ravenous in the TARDIS to be destroyed as the ship's interior dimensions finally collapsed. (AUDIO: Seizure)
The Ravenous were essentially the natural predator of the Time Lords, having evolved specifically to feed on Regeneration energy; however, Time Lords suffering from Regenerative dissonance, such as The Eleven, were essentially immune to the Ravenous, as their condition somehow contaminated their regenerative energy so that the Ravenous couldn't feed on it.