Orb of Lightning

Orb of Lightning adds 5 bonus damage to the attack of a Hero when carried. The Hero's attacks also become ranged when attacking air, and have a chance to cast Purge on the enemy. The damage to summoned units is instead 150%. The chance of purge is 35% against units, and 10% against heroes.

Source
These can be bought from the Voodoo Lounge in Frozen Throne.