Valak is a Greater Demon, also known as a memory demon.
Over a decade later, Magnus summoned him again to retrieve the memories and return it to Clary. After being summoned, Valak demanded a memory of their beloved from each one. Isabelle Lightwood surrendered a memory of her brother, the demon took from Clary a memory of her mother, and when it was Alec's turn, a memory of his parabatai Jace appeared.
This left Alec shaken and defensive, causing him to break from the bond that kept the demon contained. The demon lashed out, but Magnus managed to keep him contained, but not for long. In an attempt to keep the demon at bay, Alec approached it but was pushed aside by Jace, who was immediately gripped by the demon. Clary gave up on her desire to get her memories back to save him, easily killing the contained demon in one swoop with a seraph blade.
- 1x04: Raising Hell