The Grand Theogonist is the spiritual leader of the Cult of Sigmar, and as a consequence holds great secular and political influence over matters within the Empire. Together with his two Arch Lectors, the Grand Theogonist commands three votes on the Council of Electors, and is the person who leads the coronation ceremony of a new Emperor. This grants him immense power, since even Counts and Barons who resent the authority of the Emperor must ultimately pay service to Sigmar's representatives on earth. It has even been said on occasion, and in hushed tones, that the Grand Theogonist is the true power behind the Imperial Throne, able to manipulate the more weak-willed Emperors for the Cult's ends. Certainly this has been true in times past, although the rise of the Reikland Princes has created a more equal relationship. He is certainly one of the most important figures in the Empire.
The current Grand Theogonist is Volkmar the Grim, who has a reputation for being a stern traditionalist, but also a pragmatic and forceful leader of the Cult of Sigmar. It's worth noting that even those reformers hostile to the Cult's bureaucracy, like Luthor Huss, tend to hold Volkmar in high regard, although they are often impatient with his caution against making too-hasty reformations.
The Theogonacy was founded by Johan Helstrum, the Prophet of Sigmar, and the traditions of the office are maintained to the current day. Traditionally, the Grand Theogonist is elected by the Lectors and Arch-Lectors of the Cult of Sigmar. Upon taking the position, the Grand Theogonist assumes a Khazalid name in honor of the deep bond between Sigmar and the Dwarf people. The Grand Theogonist is privy to many ancient and forbidden secrets that have been hidden from the common people, which the Theogonists have used at times (such as the von Carstein Wars) to defend the Empire against supernatural danger. The Grand Theogonist is also the spiritual leader of the Order of Sigmar, and has a great deal of influence over the Witch Hunters.