He served as the referendarius of Duke Tassilo III (r. 748–788),[2] a position variously translated as chief counsellor,[3] secretary or chancellor.
[4] He wrote a now lost chronicle in Latin that is partially preserved by Johannes Aventinus (1477–1534), who inserted passages from it into his Annales ducum Boiariae and also translated some into German for his Bairische Chronik.
[4] Nonetheless, Aventinus occasionally amended the works he copied to give them a false precision, so his version of Creontius' text must be approached critically.
This claim is not corroborated elsewhere and is considered unlikely, but it indicates Creontius' hostility to Charlemagne and partiality to Desiderius that he tried to combat the official line on Gerperga.
[15] In his account of the extreme weather events and supernatural signs that occurred in the winter of 786—widely noted at the time—Creontius reports that Tassilo, "on the advice of the Bavarian bishops and other wise men of the land, had a general fast ordered throughout the entire land; everyone including the prince himself had to fast, to strew ashes on his bare head, to go barefoot, to do public penance in church and to make confession.