Stops all enemies in a target area from casting spells.
Kael and Lady Vashj can buy these from Arcane Vaults in Gates of the Abyss, chapter 5 of the The Frozen Throne Alliance campaign, Curse of the Blood Elves.
Akama has one in Lord of Outland, chapter 6 of that campaign.
Staff of Silence in the TCG.
Item icon in Warcraft III: Reforged.