First floor of the Wizards' Tower, in the containment unit.
Notes:
Apparently known as Azacorax, Keeper of the Key to the Abyss. Zanmaron the Red summoned him to aid in the ritual that created the power beam of the Wizards' Tower.
You can talk to the demon after completing the Rune Memories quest to receive bonus experience in Prayer or Magic if you have reached at least level 50 in both skills.