Either from Purify if I'm not wrong, or by, from the first page:
A whisper from the ruins of Tristram mentions a new device which may be the key to ending this madness. The Staff of Herding itself cannot be found in the wild, but plans to build one may be uncovered in Tristram as you save Deckard Cain. This powerful staff provides a new anti-goblin skill: Rainbow Storm. This seemingly fun looking spell is nothing if not deadly, and all players should have one in their stash, just in case... Those who decide to use the skill not only to kill goblins but as the foundation of their build, will be rewarded with ludicrous amounts of magic find. This staff holds many secrets, including the possibility to learn Telekinesis, which may be of great use this season...