The Necronomicon, also known by its true title, the Kitab al-Azif, is a grimoire of dark rituals and forbidden knowledge tied to the Outer Gods and the Outside, and penned by the Mad Arab, Abdul al-Hazred some thousand years ago. Since then, the tome has passed from dark occultist to mad cult to warlock. It never stays in one's hands for long, as those who make use of it have a bad tendency to become consumed by its terrible secrets, and attempt magic no mortal soul was meant to handle. The wise usually make a copy of a select few portions of the book, while the rest... well, their ends are as horrific as one could imagine.
These copies are what one normally finds when researching the tome--a chapter on the inhabitants of the Outside here, a ritual on blood magic there. These grimoires are often frustratingly incomplete, offering only a taste of the true power contained within the Kitab al-Azif itself. However, they also lack the tome's sinister properties, and can thus safely be contained within a library or archive without risk to the owner or their environs.
These are what have been tracked down by agents of the Veil Treaty and the Orders of Atlantis over the years--for try as they might, the Necronomicon always seems to slip through their fingers. They arrive a moment too late, only to find the cursed grimoire has been stolen by some overeager apprentice, or hidden in some cache at the other side of the world. As an example, the Veil Treaty's Men in Black were certain they would find the tome in Berlin when they raided the Thule Society's headquarters--only to find the Thule had relocated to New Swabia on the Antarctic Coast. During the Allied invasion of New Swabia, a standing order was put out to obtain the tome, but then the Thule transported their entire base to the Dark Side of the Moon, thus (apparently) bringing the book out of the reach of the Veil Treaty. Only, it had actually been found by Soviet agents, and moved the Red Library, where it was used for several years to enhance the Soviet occult program--until the Library itself was destroyed in a horrible accident and had to be nuked to stop whatever was... inside... from getting out.
Since then, records of the Kitab al-Azif have been found in the hands of extremist cults in Japan, an organized crime syndicate in the United States, a temple in India, democratic rebels in Soviet Ukraine, and an Islamic terrorist group in the Levant. Its current location is unknown, but its last known sighting was in Medan, Indonesia some 15 years ago... and signs point to its last bearer heading south and east, towards either Australia or the South Pacific.