From this matter he composed the Fasti Campililienses in two large volumes (Linz, 1747–1754): this monumental work was a history of Lilienfeld from the thirteenth century to the end of the Middle Ages, a history of the Babenberg dynasty, and also of the Steyer region.
According to the 18th century historian Chrysostomus Hanthaler, his grandson Duke Frederick II of Austria (1230–1246), nicknamed the "Quarrelsome" or the "Warlike", the last of the Babenberg dynasty, designed a new coat of arms in red-white-red after his accession—an attempt to prevail against reluctant local nobles and to stress his autonomy towards Emperor Frederick II.
He was the son of Nizzo (or Nizo) of Krems-Gars, who was burgrave on the Babenberg castles of Krems and Gars.
Sunthaym worked on a history and genealogy of the Babenberg family until 1489, perusing the histories of Otto von Freising and Thomas Ebendorfer.
Leopold III, Margrave of Austria, the Saint (1050–1136), Margrave of Austria from the house of Babenberg
In the Georgenberg Pact of 1186 he had agreed that his lands should pass to Leopold V, the Babenberg Duke of Austria.
William married Judith or Ita von Babenberg, daughter of Leopold III of Austria and Agnes of Germany, sometime before March 28, 1133.