Psionovore

The Psionovores were a race of beings which fed on fear and anxiety. Normally a gaseous or energy-based race, they could coalesce into a physical form, given sufficient will.

Biology
The Psionovores were energy-based beings inhabiting the ionised dust left by comets in interstellar space. Under normal conditions they could shape the dust into a semblance of solid objects, but maintaining those objects took significant will. With enough channelled will power, any manner of objects could be created wherever they wished, and could interact with other physical objects.

As a secondary form, Psionovores could possess the body of a sentient being, but only if the consciousness of that body was absent. To that end, they developed a number of ways to shift the minds of sentient beings.

Bad thoughts and desires were food for Psionovores. The Eighth Doctor described it this way:
 * "They feed on the negative neuronic emissions of sentient beings - pain, fear, anxiety, jealousy. A Psionovore battens on it like maggots on rotting food." (BFA: Minuet in Hell)

To that end, they would take any opportunity to promote a person's fears and anger, and then drink in the resulting psionic energy.

History
Around 2003 the newly reconstituted Hellfire Club, led by Brigham Elisha Dashwood III, attempted to summon Lucifer to do their bidding. The energy generated by them attracted the Psionovores from space and gave the creatures enough power to create Marchosias - a gestalt physical manifestation. Fitting in with the summoning, Marchosias appeared as an eight-foot-tall red demon. Marchosias proceeded to give Dashwood the secrets to building a mind-transferral machine (the PSI-859) and a trans-dimensional teleporter (the trans-D), with the intent of emptying human minds of their personalities and filling them with Psionovores instead. The Doctor's interference destroyed the machines and returned Marchosias to space. (BFA: Minuet in Hell)

The Doctor also referred in a journal entry to an altercation involving Psionovores, Edward Kelly, Christopher Marlowe, and William Shakespeare. The details of this encounter have not been related. (ST: Apocrypha Bipedium) The Eighth Doctor and Sam Jones once faced the Psionovores. (EDA: Placebo Effect)

Behind the scenes
The Psionovores were originally created for the Audio Visuals series of amateur audio plays by Alan W. Lear. While the Psionovores appeared in a number of scripts, Minuet in Hell was the only script featuring them to be adapted for publication by Big Finish Productions.