Book of Medivh
The source of much rumor and speculation both within the game world and among fans, the Book of Medivh refers to a spellbook (or possibly two or more spellbooks) written by Medivh, the Last Guardian.
Medivh's spellbook was kept in the Royal Library of New Stormwind following his death. It was retrieved by Alterac agents and taken to their kingdom. Teron Gorefiend with the help of Deathwing made a deal with Alterac king - Aiden Perenolde - and got the book in exchange for destroying Alliance outposts in Alterac. Eventually the book ended up with Ner'zhul, who also obtained the Jeweled Scepter of Sargeras, the Skull of Gul'dan, the Eye of Dalaran and used them to open many portals on Draenor. Ner'zhul's portals ripped Draenor apart. Obris held the book on the verge of death, and handed it to Khadgar. Khadgar used its knowledge to destroy the Dark Portal from Draenor's side, thus protecting Azeroth from the destruction of Draenor. But before the Portal was closed, Khadgar sent a Wildhammer gryphon rider with the Skull and the Book back to Azeroth, with orders to give them to the Kirin Tor.
The gryphon rider was successful.
Archmage Antonidas of the Kirin Tor came into the possession of the Book of Medivh and secured it in a Dalaran vault. During the Third War and the Siege of Dalaran, It was stolen from the city by Arthas and Kel'Thuzad after which, the lich used it to summon Archimonde. Archimonde referred to it as "the last surviving spellbook of Medivh, the Last Guardian." Kel'Thuzad was amazed at the amount of information about demons contained in the book.
After Archimonde and the Legion left for Kalimdor, the book remained in the hands of Kel'Thuzad.
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Kel'Thuzad apparently gave the book to his Cult of the Damned, and was stored by them inside Scholomance. On orders from Gul'dan, members of the Shadow Council led by Jergosh the Invoker retrieve it from Caer Darrow. Gul'dan intends on using the book, along with the Scepter of Sargeras and Eye of Dalaran, to open rifts through which the Burning Legion can invade Azeroth. Jergosh is killed and the book taken from him by a warlock adventurer.
One Book or Two?
There is some debate among the fan community as to whether the references to the Book of Medivh in Warcraft II: Beyond the Dark Portal and Warcraft III refer to the same book, or to different books. Archimonde referred to the copy held in Dalaran as "the only remaining spellbook of Medivh, the Last Guardian." This would seem to imply there had been others at one time. It's also possible that Khadgar's book was entitled The Book of Medivh, whereas the one acquired by Kel'Thuzad was simply a working spellbook. It has since been confirmed that the book Khadgar and Kel'thuzad used were the same book.
In Karazhan the Shade of Nielas Aran, the involuntary father of Medivh, has , which could also be considered a "Book of Medivh". Mages from the Violet Eye - a sect of the Kirin Tor sent to spy on Medivh - were looking for the book and went mad when they read it. It was revealed that Kamsis gave the book to the Shade of Aran. When the player reads the book at the Master's Terrace they can watch into the past how Medivh killed the mighty blue dragon Arcanagos, now better known as Nightbane.