Marchosias

 was a physical manifestation of the Psionovores, "summoned" to Earth by Brigham Elisha Dashwood III.

Appearance and behaviour
Marchosias appeared as an eight foot tall, bright red, horned demon to fit his role as an "agent of Lucifer" for the Hellfire Club. While superficially subservient, Marchosias took any opportunity to promote fear, anxiety, and anger in anyone he met. He was capable of appearing and disappearing at will, summoning lesser demons (imps and the like), and could possess humans.

In reality, Marchosias was a gestalt manifestation of the Psionovores — a race of energy creatures that fed on negative emotions and thoughts. (AUDIO: Minuet in Hell)

Biography
Brigham Elisha Dashwood III, as head of the Hellfire Club, attempted to summon Lucifer to Earth in a bid for power. Marchosias appeared, filling in the image that the Club members wanted. He helped Dashwood to build the PSI-859 Psionic Matrix Facsimile Regenerator (a mind recorder/eraser) and the trans-D (a pistol that teleported people to the Psionovores' home space). With these devices, Marchosias hoped to empty humans of their minds, providing ready-made vessels for the Psionovores. (AUDIO: Minuet in Hell)

Behind the scenes

 * Marchosias and the Psionovores have made only one appearance in the Big Finish Doctor Who audio stories (discounting a metatextual cameo advertising the Eighth Doctor). Likewise, with their amateur predecessors, the Audio Visual dramas, appearing solely in Cloud of Fear.
 * Marchosias is named after one of the traditional commanders of Hell, said to lead "30 legions of demons" and could be easily summoned.
 * The voice actor for Marchosias is not credited, though the assumption is that he is voiced by Morgan Deare, who provides the voice for Senator Waldo Pickering — a character that Marchosias possesses for much of the story.