Spider Silk Broach

Binds a target enemy air unit in webbing, forcing the target to the ground. Webbed units can be hit as though they were land units. Contains 4 charges.

This large silver brooch looks as though it has been woven completely from spider silk, ecept for eight sharply jointed arms that extend outward like those of an arachnid. No pin or chain is required to wear this item- it attaches itself to any clothing and clings there until the owner wills it to let go.

Source
Kael and Lady Vashj can buy these from a Goblin Merchant in The Crossing, the secret level of the Frozen Throne alliance campaign, Curse of the Blood Elves.