Orb of Frost

Orb of Frost adds 6 bonus cold damage to the attacks of a Hero when carried. The Hero's attacks also slow the movement speed and attack rate of the enemy for 3 seconds.

Source
Arthas and Muradin Bronzebeard find one in The Shores of Northrend, chapter 7 of the Warcraft III human campaign, The Scourge of Lordaeron.