Valthek is a famous and very powerful sorcerer of the Vizjeri mage clan.
He tried to confront the character of the Wizard class in Diablo III, but she managed to trick him with spells and wit in order to render him unconscious. She seemed to have made him use is powerful spells, and defeat him with far less power.
Valthek seems to be one of the Vizjeri sorcerers with access to the Ancient Repositories.