Maul of Strength

Increases the Strength of the Hero by 1 when worn.

This unusually large mace appears at first glance to be virtually unusable, but it is actually quite well balanced. The added weight inside the headpiece produces devastatingly effective blows in combat.

Source
Arthas finds one in Ravages of the Plague, chapter 3 of the Warcraft III human campaign, The Scourge of Lordaeron.

Maiev can find one in The Broken Isles, chapter 2 of the Frozen Throne sentinels campaign, Terror of the Tides.

Kael can find one in Misconceptions, chapter 1 of the alliance campaign, Curse of the Blood Elves.