The source of 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.
Beyond the Dark Portal
Medivh's spellbook was kept in the Royal Library of New Stormwind following his death. Ner'zhul stole this book during the battles that took place following the Second War and used it to open many portals on Draenor. Ner'zhul also obtained the Jeweled Scepter of Sargeras, the Skull of Gul'dan, and the Eye of Dalaran. His 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.
Archmage Antonidas of the Kirin Tor apparently came into the possession of an artifact known as the Book of Medivh sometime before the Third War. It was stolen from Dalaran by Arthas and Kel'Thuzad during that war and used to summon Archimonde, thus beginning the Reign of Chaos. 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.
The book was last seen in Kel'Thuzad's possession.
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.
In Karazhan the Shade of Nielas Aran, the involuntary father of Medivh, has Medivh's Journal, 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.
There are several places where the Book of Medivh might show up:
- The book may be the same one that Kel'Thuzad retrieved from Antonidas. In that case, either Kel'Thuzad or some other Scourge agent could still hold the book.
- As Khadgar survived the destruction of Draenor and is still alive in Shattrath City, the Book of Medivh may still be in his hands.
- Lower Karazhan: This is a mirrored version of Karazhan not yet implemented in the game, but it could be that the book can be found there.