Mana Stone

Increases the mana regeneration rate of the Hero by 25% when worn. Can be consumed for 300 mana.

Source
Arthas and Kel'Thuzad can find one in Blackrock & Roll, Too!, chapter 6 of the undead campaign, Path of the Damned.

Tyrande can find one in Daughters of the Moon, chapter 2 of the night elf campaign, Eternity's End.

Illidan can find one in A Destiny of Flame and Sorrow, chapter 6 of that campaign.

Maiev can find one in Broken Isles, and one in Wrath of the Betrayer, chapters 2 and 4 of the Frozen Throne sentinels campaign, Terror of the Tides.

Kael and Lady Vashj can find one in A Dark Covenant, chapter 2 of the alliance campaign, Curse of the Blood Elves.

Kael and Lady Vashj can also find one in The Crossing, the secret level that campaign.

In the RPG
This crystalline sphere is a translucent, smoky black color. Inside a turquoise liquid seems to swirl about, as if waiting to be released. It owner can easily break it open with only the slightest pressure from his fingers.