Periapt of Vitality

Periapt of Vitality increases the hit points of the Hero by 150 when worn.

This golden amulet bears a large, raised rune over a deep-red ruby that is warm to the touch.

Source
This item can be bought from Goblin Merchants.

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

Arthas and Muradin Bronzebeard find one in Frostmourne, chapter 9 of that campaign.

Arthas and Kel'Thuzad can also find one in Blackrock & Roll, Too!, chapter 6 of the undead campaign, Path of the Damned.

Thrall and Cairne Bloodhoof can find one in Where Wyverns Dare, chapter 6 of the orc campaign, The Invasion of Kalimdor.

Tyrande can find one in Brothers in Blood, chapter 5 of the night elf campaign, Eternity's End.

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

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

Illidan, Kael'thas, Lady Vashj and Akama can find one in Lord of Outland, chapter 6 of that campaign.