Morch'aliach

Morch'aliach was an eldritch entity from another dimension.

Nature
Its true form was that of a small worm, but when at full power, it could grow to encompass a small pocket realm centred around its full form, a "tree" of interlocking wormds with a single great eye which had four pupils. The entity was sustained by the psychic energy generated by people's traumatic memories of the deaths of others. To draw these out, it would summon the souls of the dead and give them a ghoulish physical form as corpse-like puppets whose flesh was riddled with holes through which the worms that made up Morch'aliach circulated. (PROSE: A Honeycomb of Souls)

Biography
It first entered the universe in King Arthur's time; Merlin had Arthur send three of his Knights, Bedevere, Agravaine and Gawain, teaum up with Merlin's sister Ganeida to defeat the entity. However, after an initial encounter during which they had to retreat from the cave where it had made its lair, Agravaine took it upon himself to cause a rockslide that sealed the creature alive in the cave.

Knowing the creature would reemerge, Merlin sent Ganeida to the 21st century, where he advised her to seek out P.R.O.B.E. to help her. To her shock, she found that a much older Bedevere, now a kind of half-real "ghost", was a member of the team. Bedevere, Ganeida and the team made their way to the basement of an ordinary house, which was built on top of the Arthurian cave, and whose inhabitants had disappeared as Morch'aliach grew in strength once again.

Though Morch'aliach attempted to use its powers on the team, Bedevere managed to stab it with his broadsword. While this was not in and of itself successful, it allowed Ganeida to exploit the sword's link to Camelot to force Morch'aliach to summon the souls of King Arthur and the Knights, not as a memory of trauma but of valour and glory. The ghostly knights beat down Morch'aliach until it was reduced to a single wriggling worm, which Ganeida crushed underfoot. (PROSE: A Honeycomb of Souls)