Churchman Olaus Magnus' handwritten account of the Stockholm bloodbath has been found in a unique and previously unknown copy of the Danish chronicle Gesta danorum. The Gesta danorum ('The exploits of the Danes') was written by the historian Saxo Grammaticus, who worked in Denmark in the 12th century. The work itself is one of the most important sources of early Danish and Scandinavian history, but the copy of the first edition from 1514 acquired by KB is also of great importance for Sweden. It was used by the brothers Olaus and Johannes Magnus, who later published their own important works on Swedish history.

Extensive marginal notes in Latin reveal which parts of the Gesta danorum particularly interested the two history-writing brothers. Even more remarkable is that the book contains Olaus Magnus's handwritten account of the Stockholm bloodbath of 1520 - probably one of the earliest sources for the event. It can be related to a similar passage in his great work 'History of the Nordic Peoples'. Both Olaus and his brother John, who published 'The History of all the Kings of the Goths and Swedes', also refer frequently to Saxo Grammaticus in their works. In 1519, their working copy was donated to the Birgitta House in Rome. After that, the book ended up in a Carthusian monastery in Avignon. During the French Revolution, the monastery's library was confiscated, and the volume probably appeared on the book market in the mid-19th century.