Spider Ring

Increases the Agility of the Hero by 1 when worn.

This platinum ring appears to have been made of thin strands of spider silk woven in a concentric pattern. Elven garnets arranged like a spider's eyes stare out from a web-shaped faceplate.

Source
Arthas can find one in Trudging through the Ashes, chapter 1 of the Warcraft III undead campaign, Path of the Damned.

Tyrande and Furion can find one in Brothers in Blood, chapter 5 of the night elf campaign, Eternity's End.

Maiev can find one in Rise of the Naga, chapter 1 of the Frozen Throne sentinels campaign, Terror of the Tides.