Azorthragal

Azorthragal is a demon. It is first mentioned in Grave Peril.

Description
Azorthragal was the Demon that Leonid Kravos made efforts to secure as his servant.

In Leonid Kravos's journal, called Book of Shadows, Kravos had written the exact pronunciation of Azorthragal name for the spell.

Grave Peril
In Grave Peril, using a ritual spell in the copper Circle his lab, Harry attempts to summon Azorthragal, ending up summoning the Nightmare and assuming Azorthragal is the Nightmare. Harry binds the Nightmare to him so he can't hurt anyone else forcing it to come after Harry only.