A Sith spell, Agony of the Furies involves aggravating every pain receptor in the target's nervous system simultaneously - resulting in sensations of the most exquisite, profound pain arcing all through the target's body. Though the target is technically not hurt when this technique is used, Agony of the Furies very rapidly results in the subject going into shock, rupturing blood vessels (occasionally resulting in stroke) and, in some cases, suffering permanent brain damage or post-traumatic stress.
Due to the complex nature of this ability (it is not easy to simultaneously influence every nerve in someone's body, and falling anywhere short of this comprehensiveness is not considered a full application of Agony of the Furies), only masters of the Sith teachings are typically competent enough to perform it. Only one Sith Lord, Seth DeSchaen has ever managed to simultaneously use Agony of the Furies on more than one target. If other cases exist of Sith Lords being able to influence more than one person with this ability, they are not documented.