The Sacred Mallet is one of the few curious items in World of Warcraft Classic. This item is used to obtain the Mallet of Zul'Farrak, which in turn can be used in Classic Zul'Farrak to summon Gahz'rilla.
In order to get the Sacred Mallet, you ought to go to The Hinterlands, where you want to find Qiaga the Keeper, who can be found at the top of the Altar of Zul, close to the Jintha'Alor Altar.
At the altar, you should find three normal mobs and two elites, one of which is going to drop the item. Since they are elites, bringing a friend or two is advisable, since killing them on your own might not always be easy.
After using the Sacred Mallet at the Jintha'Alor Altar, you will get the proper item that will allow you to summon the last boss of the Zul'Farrak instance.