Anti-magic Potion

Gives the Hero immunity to magical spells for 15 seconds.

Source
These can be bought from Ancients of Wonders in The Frozen Throne.

Arthas and Kel'Thuzad can find one in The Siege of Dalaran, chapter 7 of the undead campaign, Path of the Damned.

Grom Hellscream can find one in The Hunter of Shadows, chapter 5 of the orc campaign, The Invasion of Kalimdor.

Tyrande can find one, and buy these from a Goblin Merchant, in The Awakening of Stormrage, chapter 3 of the night elf campaign, Eternity's End.

In the RPG
This powerful item looks like a vial filled with a clear liquid that appears at first glance to be merely water. If viewed carefully, however, small flecks of gold can be seen swirling about within. When unstopped, the potion begins to bubble and seethe like a cauldron set to boiling.