Wilhelm III was the Grand Theogonist during the time of the First Vampire War, who was instrumental in the destruction of Vlad von Carstein, the First of the Vampire Counts. Little is known of Wilhelm other than the fact that he was present during the Siege of Altdorf in 2051 IC. In search of divine aid, the Grand Theogonist retreated into the catacombs of the Cathedral of Altdorf to pray. It was in that holy place that he received a visit from Mannfred von Carstein, the eldest thrall of Vlad.
Mannfred wanted the power of the Vampire Count for himself, but he knew that he wasn't strong enough to challenge Vlad in combat. So he informed Wilhelm III about the secret of the Count's immortality. The priest then set out an elaborated trap to catch the best thief of this very age: Felix Mann. When they finally caught him robbing the Imperial Counting House, they gave him a simple choice: to rob Vlad von Carstein's signet ring, or to be hung for his crimes. In reward of the theft, he would be paid enough to start a living elsewhere. Felix accepted the theft and, with a little aid from Mannfred von Carstein, robbed the Count of his most precious belonging.
When the Count awakened, his anger was uncontrollable. In one frenetic attack, Vlad threw everything he had against Altdorf. There on the battlements, he confronted Wilhelm III and eventually, overcame him. However, with his last breath, the Great Theogonist threw himself against the Undead Count, and both fell off the walls. While the fall wouldn't have killed a Vampire as powerful as Vlad, they both fell on a wooden stake, the Vampire trapped beneath the corpse of the holy man. With the death of Vlad, his army dissolved and the Altdorfers rejoiced, for finally the horrors of the First Vampire War were over.