Orb of Fire

Adds 5 bonus fire damage to the attack of a Hero when carried. The Hero's attacks also become ranged when attacking air and do splash damage to nearby enemy units.

Source
These can be bought from Arcane Vaults in Frozen Throne.

Arthas receives one in Blackrock & Roll, chapter 2 of the Warcraft III human campaign, The Scourge of Lordaeron, after delivering The Heart of Searinox to Feranor Steeltoe.

Grom Hellscream finds one in The Hunter of Shadows, chapter 5 of the orc campaign, The Invasion of Kalimdor.

Thrall finds one in The Oracle, chapter 7 of that campaign.

Tyrande and Maiev can find one in Shards of the Alliance, chapter 6 of the Frozen Throne sentinels campaign, Terror of the Tides.