The Glass Goblet is an item from Witchery. This item is used to collect blood for vampiric practices. It is used in the Creatures of the Night ritual in order to initiate the Vampirism gameplay. It is also used by level 9 vampires to turn other players or villagers into vampires through the Create Vampires ability.
Drinking a Glass Goblet full of blood will provide the Weakness IV, Slowness, and Mining Fatigue effects for about 10 seconds.