The Necronomicon is a powerful and perilous magical tome of ancient origins. It was originally written by Abdul Alhazred, a great magic-user known to some as "the Mad Arab". After ten years alone in the desert he wrote a book called Al Azif - words used to denote the nocturnal sounds of insects which may be the voices of demons. Alhazred was later seized in the streets of a desert city by invisible demons and devoured horribly in front of many witnesses. Those who have studied his writings have sometimes met a fate nearly as terrible. The book was banned and all known copies were destroyed, but a few translated editions, retitled The Necronomicon, still exist.
The book gives a description of the pre-human worship of the Old Ones, their banishment by the elder gods, and their imminent return. The revelations of cosmic horror contained within its pages are so intense that there is a probability of reader going mad.
It would appear that spells are given for summoning all of the Old Ones and their minions, and some spells for their control and dismissal, although these latter are not always effective. The spells are very long and complicated, and not entirely comprehensible without long study and research. In fact, only highly experienced magic-users have a chance of understanding (and thereby using) them. Many of the spells require that the "stars be right", and can only take place at certain times of the year or in certain places. Attempting to cast any of the greater summoning spells will result in a chance of the caster going insane. The spells don't always work: in particular they often fail to protect the magic-user from the thing he or she has called from the outer darkness. Such unfortunates are rarely seen again, although simulacra or zombie-like imitations sometimes appear.
In addition to The Necronomicon, there are a number of other magical books giving information on the Old Ones and their minions, including: The Book of Eibon, the Celaeno Fragments, Cultes des Goules by the Comte d'Erlette, De Vermis Mysteriis by Ludvig Prinn, the Dhol Chants, the Pnakotic Manuscripts, and Unausprechlichen Kulten by Von Junzt. Most of these are histories, but some have powers similar to, but lesser than, The Necronomicon itself. These lesser books are not as dangerous to the magic-user or his surroundings.