Ring of Protection +5

Increases the armor of the Hero by 5 when worn.

Source
Arthas can find one in Under the Burning Sky, chapter 8 of the Warcraft III undead campaign, Path of the Damned.

Illidan can find one in The Brothers Stormrage, chapter 8 of the Frozen Throne sentinels campaign, Terror of the Tides.

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