Staff of Negation

Dispels all magical effects in a target area. Deals 200 damage to summoned units. The radius is 20.

Source
These can be bought from Goblin Merchants in some campaign missions 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.

In the RPG
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.