Orb of Slow

Adds 6 bonus damage to the attack of a Hero when carried. The Hero's attacks also become ranged when attacking air, and have a change to slow a target enemy unit's movement speed by 60% and attack rate by 25% for 60 seconds.

Source
Maiev can find one in Wrath of the Betrayer, chapter 4 of the Frozen Throne sentinels campaign, Terror of the Tides.

Kael and Lady Vashj can find one in The Crossing, the secret level of the alliance campaign, Curse of the Blood Elves.