The Hervoken were an ancient species.
The Hervoken were approximately ten feet tall, with an extremely thin, black body but a colossal, wide head that somewhat resembled a Halloween jack-o'-lantern. Their hands were about as long from the wrist to the fingertips as a human arm, and their fingers had several joints and a talon at the end. Their heads were pale and fleshy, with sharp teeth within a wide, jagged, lip-less mouth and black, deep-set, beady eyes.
Hervoken had high-pitched, almost childish voices and their laughter was made up of clacking their teeth together to produce green sparks. They are clothed in a rotting patchwork of black, tattered rags and appear to slightly hover above the ground. (PROSE: Forever Autumn)
Hervoken were insular by nature and tried not to deal with others if they could. They had little of the curiosity that humans had and were content in simply keeping to themselves. Because of this, they seemed remarkably callous in their nature.
The Hervoken language was complex and consisted of words, body language, thoughts, incantation and pauses. As such, simple acts like chanting, resting or praying were actually methods of communication. (PROSE: Forever Autumn)
Hervoken relied on agents, which were created by animating inanimate matter and modifying existing beings. These agents would be able to do new things, but they would not change in basic structure (a plastic bat may be able to fly and attack, but it would still be plastic).
The Hervoken were an ancient species, though they kept to themselves for most of the time. At some point, millions of years ago, they got into a war with the Carrionites. The details of this war were vague, but it was implied to have been very destructive, as the Eternals were forced to step in and banish both races to the Deep Darkness before the universe was destroyed.
One Hervoken ship escaped, since it was on Earth at the time, and escaped the notice of the Eternals. The ship lay dormant for many years, attracting people to the area to maintain it. They soon passed into the myths of the Niantics, being known as the Hee-oko. Eventually, the town of Blackwood Falls was built on top of the Hervoken ship. The Hervoken were able to psychically influence the inhabitants, making them create cellars into the Hervoken ship so they could be used for fuel. The side-effect of this was that Etta Helligan's family developed psychic powers.
Eventually, the ship repaired itself completely. When the Hervoken were reawakened in the early 21st century, they planned to repower their ship with energy from the town's inhabitants during the upcoming Halloween celebration. The launch of the ship would have devastated the town and most of the surrounding area, but the Hervoken were defeated by the Tenth Doctor and Martha Jones. (PROSE: Forever Autumn)
When Vida Clatterly expressed the belief that magic was not real, the Eleventh Doctor told her about the Hervoken, among others, who used science disguised as magic and explained that belief is a very powerful thing. (PROSE: The Dreaming)