Adalbert Vojtech of Prague had already in 977 entertained the idea of becoming a missionary in Prussia. After he had converted Hungary, he was sent by the pope to convert the heathen Prussians. Boleslaw I Chrobry, duke of Poland sent soldiers with Adalbert. Adalbert and his followers entered Prussia territory near Gdansk and went along the Baltic Sea coast.
It was a standard procedure of Christian missionaries to try to chop down sacred oak trees (see Iconoclasm), which they had done in many other places, including Saxony. Because the trees were worshipped and the spirits who were believed to inhabit the trees were feared for their powers, this was done to demonstrate to the non-Christians that no supernatural powers protected the trees from the Christians.
When they did not heed warnings to stay away from the sacred oak groves, Adalbert was martyred April A.D. 997 near later Fischhausen near the Nogat river. It is said that his body was bought back for its weight in gold by Boleslaus I of Poland. This investment perfectly paid off.
A few years later Adalbert was canonized as Saint Adalbert of Prague. His life has been written about in 'Vita St Adalberti' by various writers, the earliest was traced to imperial Aachen and Lüttich, although it was assumed for many years that the Roman monk John Canaparius had written the first 'Vita'.
In 1037 Czech King Bretislav I retrieved the bones of Saint Adalbert from Gniezno and moved it to Prague.