Staff of Negation

Dispels all magical effect in a target area. Deals 200 damage to summoned units.

This staff of polished gray wood is cold tot he touch and inlaid with black runes. Bands of silver bind a star-shaped crystal to its tip.

Source
These can be bought from Goblin Merchants in the Frozen Throne.

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