Rock Demons are antagonists in Paul Stewart and Chris Riddell's books The Edge Chronicles. They are monsters which live in the old dockyards of Undertown. They were created by forces of nature and rapid evolution after the Edge changed dramatically when the Mother Storm came and Stone-sickness arrived.
The Rock Demons were given Librarians to eat, by Orbix Xaxis. He called them "Demons of the Deep." In his view, sacrificing Librarians was like a ritual to get the Storm to arrive again.
The Rock Demons were all drowned when the Storm arrived and the sewers flooded.
When the Stone-sickness came the Edge changed. The city of Undertown had catastrophic changes; earthquakes hit and buildings collapsed, forming a series of chasms around the dockyard. This became a place of fear for the residents who believed it to be full of evil spirits and demons. The Librarians saw strange creatures during their flights there. Sometimes, Librarians would go missing, taken by the Guardians of Night. After an interrogation, they would be shut in cages and lowered into the pits, being fed to the Demons.
The Rock Demons were grotesque, bat-like creatures, which had hairy faces, leathery hooked wings, two clawed legs, and a long probiscis on their snouts. They presumably sucked out their victims' blood with this probiscis. They were black, and had yellow eyes which could be seen when they attacked at night. They were also called "cleansers of the Sky" by Orbix, who believed the Librarians were polluting the Sky with their flight.
In the series
The Rock Demons were seen in book eight, Vox, when Orbix interrogates two Librarians and sentences them to death. He says "Will you never learn that it is us, the Guardians, who are the masters?" He then orders them to be imprisoned and lowered down to the pits, and released to be eaten. The horrible Rock Demons come out and immediately devour both the frightened Librarians.
The Rock Demons are always hungry, and Orbix decides to starve them for a while, and keep them hungry, so when he finally releases them onto Librarian bait, they'll devour them all. He has his men dig tunnels from the pits into the sewers, where he drove the Librarians. He then captures Magda, a Librarian, and has Xanth torture her. But Xanth hates the Guardians and he abandons them to free Magda. They leave and try to run, but are captured.
The Rock Demons come out and hungrily chase the pair but lose them in the sewers. The Rock Demons, instead, find another food source - the goblin and shryke armies, who were fighting in the sewers. The Rock Demons hungrily devour them all, but are soon drowned by the floods from the maelstrom.