The Ridderak is a type of Valg. Offering it sacrifices allows a person to gain the powers of its victims.
Ridderaks are 'vaguely human' in appearance. They have dark eyes and reeking black blood, and their grey skin is tight and hairless. They have misshapen heads with slit-like nostrils and mouths filled with venomous black fangs. They have long front arms, and claws sharp enough to carve stone. Their knees are like those on an animal's hind legs, and they appear to be quadrupedal. They are surprisingly lightweight.
Ridderaks are bloodthirsty and vicious killers. In a ritual sacrifice, a Ridderak will slice open the victim's chest cavity and devour their internal organs, and drill a hole in the crown of the victims's head and eat their brain; their victim is usually missing skin from their face.
Despite their predatory, ruthless nature, Ridderaks will submit to the person who summons them in order to conduct the ritual sacrifice. In this sacrifice, the summoner incapacitates a victim and writes Wyrdmarks in the victim's blood, which summons the Ridderak. The Ridderak then feasts on the victim's flesh and, in return, grants the summoner the victim's power.
Celaena later searches for Nehemia Ytger in the tunnels, fearing that she is behind the killings, but instead encounters Cain summoning the Ridderak. He locks her in the room with the Ridderak and bids her goodbye.
In a last ditch plan to survive, Celaena throws her cloak on the floor, and as the Ridderak leaps for her, she sails over it; the Ridderak lands on her cloak and slides into the door, smashing it so that she can escape the room. As she runs to Elena Havilliard's tomb, the Ridderak pursues her. She swings into the tomb and grabs Damaris. The beast lunges for her again; it bites her hand at the same moment she impales it with Damaris. Celaena leaves its corpse in the tomb and thanks Elena before returning to her chambers, where Nehemia finds her as she succumbs to the venom of the Ridderak's bite.
- It is unknown whether Ridderaks are capable of speech.