The Book of Medivh

After Medivh’s death his spellbook was kept in the royal library of rebuilt Stormwind, until agents of Alterac stole it. Through a bargain between Alterac’s king and the Horde death knight Teron Gorefiend, it passed to the orcs, and on Draenor Ner'zhul used it with other relics to open new portals, rifts that tore his world apart. Before the Dark Portal fell, Khadgar recovered the book and sent it home by gryphon.

The Kirin Tor sealed it in a vault in Dalaran. During the Third War, Kel'Thuzad shattered that vault, and with the book’s power he opened the rift through which Archimonde and the Burning Legion entered Azeroth. Afterwards the book stayed in the lich’s hands.