Shax are parasitic breeding demons.
Shax demons have great pincers and an insect-like carapace for armor.
Known for their keen sense of smell, Shax demons are sometimes summoned by warlocks to track, pursue, or hunt down a missing person. They must be carefully controlled because, being brood parasites, they tend to use their targets to breed. They reproduce by wounding a victim, dragging them back to their lair, and then laying their eggs in the still-living and conscious victim's skin.
In 1878, a Shax demon was sent by the Dark Sisters to find and bring Emma Bayliss back to them. The demon succeeded, and Emma gave chase while she bled to death from a previous injury. The demon was later killed by Will Herondale. By then, Will had already been encountering several Shax demons; to counter this, he ordered a stock of misericord blades—narrow blades that could pierce the demon's armor—for the Institute.
Some time before Will's death in 1937, he was bitten by a Shax demon and had nearly died. His former parabatai Jem Carstairs, then Brother Zachariah, arrived and sat with him all night to ensure his recovery. Jem later recalled this story when he played his violin on Will's deathbed.