The Valg are a race of powerful and malicious demons. They created the Wyrdkeys in order to enter other realms.
After the First War, many of the Valg were either killed or exiled, but a few remained in Erilea, including Erawan, one of three extremely powerful Valg Kings. Gavin Havilliard and Elena Galathynius fought Erawan a second time centuries later and, using the Lock forged by Mala, they managed to contain Erawan in a sarcophagus in Morath, which could only be opened by using a Wyrdkey.
During their time in the world, the Valg kidnapped and mated with the Fae, producing both the Ironteeth witches (who take more after their Valg parent) and the Crochan witches (who take more after their Fae parent).
All Valg encounters thus far have displayed unbridled malice and love of inflicting pain. Their expression of these traits, however, differ depending on the variety of Valg in question. The Valg princes specifically are cold and calculating, and they delight in emotional pain as well as physical. Four Valg princes forced Aelin to relive her worst memories during the attack on Mistward in Heir of Fire.
Abilities vary by the type of Valg demon.
All Valg however have an aversion to fire. The King of Adarlan revealed in Queen of Shadows that Erawan tried to wipe out the Galathynius line completely in hopes that Aelin would never be able to destroy them. Using her fire, Aelin successfully killed General Narrok and three other Valg princes.