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