Based on dubious sources, Simon's popular Prussian Chronicle has been criticized as a work of fiction, however, Grunau himself claimed to have incorporated information from an ancient chronicle penned by the first Prussian Bishop (d. 1245).  According to legend, the brothers Widewuto and Bruteno were kings of the Cimbri, who were driven from their homeland by Goths and so migrated to the primitive land of Ulmiganea. Widewuto and Bruteno taught the people there agriculture and civilized the region, naming it Prussia after Bruteno (Pruteno).  As king, Widewuto ruled wisely and issued laws regulating family life, public life, and criminal law. Bruteno served as his high priest (criwo cyrwaito) in charge of religious life. Widewuto had 12 sons, whose names are immortalized in the 12 districts of Old Prussia:  Lithuania was named for the king's eldest son Litvas, Sudovia was named for his son  Sudo, and so on. At the age of 116, Widewuto and Bruteno immolated themselves in a religious ceremony in the temple of Romuva, where the brothers were then worshiped as the god Wurskaito. They are known from writings of 16th-century chroniclers Erasmus Stella, Simon Grunau, and Lucas David.

The Widewuto flag, which Grunau reproduced in his work, bears portraits of three Prussian gods, accompanied by a series of mysterious symbols, somewhat similar to Cyrillic. On the left is Peckols, the god of the underworld, an old man with white beard and a white scarf covering his head. In the center sits the thunder god, Perkūnas, with fiery hair. Patrimpas, the god of crops, portrayed as a beardless youth wearing a wreath of grain, appears to the right.