The Behemoth is a huge, formless demon that is semi-corporeal and considerably difficult to kill.
The Behemoth is a formless monstrosity much larger than a human, roughly oblong in shape and has a double row of teeth lining the length of its body. It has a ridged underside like a cockroach's. A Behemoth devours everything in its path, including people. When its serrated jaws clacks together, the Behemoth makes low, insectile chittering sounds.
It is slimy and composed chiefly of a viscous, mucus-like substance. It slithers in a slow, oozing, boneless motion comparable to that of a slug's, but with less coherence of shape. The Behemoth is also capable of quickly dispersing and reforming elsewhere in the vicinity.
Behemoths are hard to kill. Not only does its slime render seraph blades ineffective, but it also makes them capable of healing its wounds and reforming. When it is attacked and seemingly destroyed, the Behemoth goes through convulsive spasms and shudders violently before reforming. This process, however, seeps its energy, so a continuous chain of attacks until it can no longer heal itself is the most effective way of destroying it.
In 1878, the brothers Gabriel and Gideon Lightwood, during their mission to disgrace Charlotte Branwell, wrote letters to Consul Wayland where they pretended to have been too weak to defeat a Behemoth demon, which they would normally be able destroy in large numbers easily.
In 1929, a group of Behemoths was summoned by the warlock Aldous Nix during his attempt to use the Hotel Dumort as a Portal to the Void. They slaughtered the wealthy mundanes who had given Aldous the hotel and those who had come to see a demonstration of his power. They were presumably dispatched by the New York Shadowhunters.
In 2007, a Behemoth was among the demons summoned to the vulnerable Alicante. Clary and Jace encountered one lurking in a cake shop. As it was the first time Clary had ever encountered one, she hid behind Jace in fear and disgust. The pair tried to escape after it first reformed, but the demon blocked their path to safety. She watched as Jace pummeled it with his seraph blade, which the Behemoth just later put out with its mucus. Clary tried to interfere, thinking that wounding the demon enough times might tire it out and prevent it from reforming any further. The werewolf Maia then appeared and tore at it with her teeth and claws. While the Behemoth desperately struggled to heal and reform, Maia never gave it the chance and eventually disintegrated, leaving behind a viscous puddle of green fluid.