Orb of Venom

Adds 5 bonus damage to the attack of a Hero when carried. The Hero's attacks also become ranged when attacking air and poison enemy units for 10 seconds.

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

Illidan, Kael, Lady Vashj and Akama can find one in Lord of Outland, chapter 6 of the Frozen Throne alliance campaign, Curse of the Blood Elves.