Cerberus demons are watchdog creatures that resemble a mix between a tree and a frog.
Cerberus demons are described as dark and twisting like a mangled tree root with frog-like legs. They have black eyes, claws, and a fanged mouth with thorn sharp brown teeth and a long, black, sticky tongue. Their bodies are coated in black slime and they hop to get around.
This variation of Cerberus demon—having been bred with some kind of demonic plant and left in a greenhouse for decades to mutate—has dark talons and membranous wings with pulsing red veins branching off from its central stalk. It has three heads and each resembles a wolf's but with black insectile eyes. It also has squid-like tentacles coming from the bottom clustered with membranous seedpods.
Before his death, Benedict Lightwood crossbred a Cerberus demon with a kind of demonic plant so that when slain, it would drop pods that would then hatch and grow into new Cerberus demons. He placed it in his greenhouse and instilled it with the mandate of protecting his family which would be passed down through its progeny as they would remember all their progenitor knew. He also summoned and placed around a dozen more Cerberus demons around his home in Chiswick to protect his family. When searching his property after his death, they were mostly all found and destroyed except for one in the greenhouse.
Over years of living in the greenhouse, the demon began to adapt and change in order to blend in better with its surroundings. By 1903, when it was encountered by Cordelia Carstairs and James Herondale, it was nearly unrecognizable. Cordelia managed to cut one of its tentacles off from around her leg but as they tried to flee, the demon blocked the exit. James then quickly dispatched it and the pair left paying no attention to the seedpods it had dropped before vanishing.
These pods hatched overnight into many Cerberus demons and grew to their full size in a matter of hours. When Grace Blackthorn snuck out from her house, they went to retrieve her, as is their mandate. She and James were attacked at the Battersea Bridge by the mass of demon spawn, but luckily were joined by friends and able to begin fighting them off. Toward the end of the battle, the largest one spoke to James and taunted him about his demonic grandfather. He responded that if they truly worshiped his grandfather, they would return to their dimension in his name. The demons obliged and all vanished.