Shax are parasitic demons.
Shax demons have great pincers and an insect-like carapace for armor. According to Jace, they are considered "the bloodhounds of the Shadow World," often summoned to track, pursue, or hunt down a person.
A horde of Shax demons were also sent to track Jace, Clary, Isabelle, and Alec when they went to the NYPD station to retrieve the Mortal Cup. The demons followed them around the tunnels of the building they entered, and the Shadowhunters decided to split up in an effort to make it harder to track them and draw the attention away from Clary, who had the Cup. Despite this, three Shax demons were able to follow and corner Clary, who was able to use the Cup to force the demons to flee.
When Valentine Morgenstern found out from his mole at the Institute that Ragnor Fell held the key to Jocelyn's awakening, he sent Shax demons after them. A Shax demon was able to follow Jace, Clary, and Magnus through Ragnor's wards and, catching Ragnor by surprise, was able to kill the warlock.
Some time after, Alec slaughtered a horde of Shax demons in Grand Central.