Medallion of Courage

Increases the Strength and Intelligence of the Hero by 4 when worn.

Very few of these medals still exist; they were once given as awards to the most celebrated heroes of Lordaeron. They take the form of a 4-pointed star on a field of blue and hang around the neck on a red-and-yellow ribbon. Since these items are considered to be military decorations in the human army, anyone wearing one of these medals without authorization will be frowned upon at best, and may find themself arrested and the item impounded at worst.

Source
Arthas can find one in March of the Scourge, chapter 5 of the Warcraft III human campaign, The Scourge of Lordaeron.

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

Thrall and Cairne Bloodhoof can find one in By Demons Be Driven, chapter 8 of the orc campaign, The Invasion of Kalimdor.

Illidan can find one in A Destiny of Flame and Sorrow, chapter 6 of the night elf campaign, Eternity's End.

Kael and Lady Vashj can find one in The Dungeons of Dalaran, chapter 3 of the Frozen Throne alliance campaign, Curse of the Blood Elves.