Permanently increases Intelligence.
Source
Arthas and Kel'Thuzad find two in Blackrock n' Roll Too!, and one in The Siege of Dalaran, chapters 6 and 7 of the Warcraft III campaign, Path of the Damned.
Thrall can find one in Cry of the Warsong, and one in The Oracle, chapters 3 and 7 of the orc campaign, The Invasion of Kalimdor.
Maiev can find one in Rise of the Naga, three in The Broken Isles, and one in The Tomb of Sargeras, chapters 1, 2 and 3 of the Frozen Throne night elf campaign, Terror of the Tides.
Kael can find two in Misconceptions, chapter 1 of the alliance campaign, Curse of the Blood Elves.
Kael and Lady Vashj can find one in The Dungeons of Dalaran, chapter 3 of that campaign.