„He had obtained a sword from the angel, by which, with God's help, defeated all his opponents. This sword till this day is kept in the treasury of the Krakow church. Polish kings going on war, used to carry it and always triumphed over the enemy. (...) the sword of King Boleslaw, given to him by an angel, is called Szczerbiec, because on the call of an angel, he came to Rus and first struck him the Golden Gate, which closes the Kiev castle. From this hit the sword suffered a small loss, which in Polish is called szczerba and hence the name Szczerbiec.” - Kronika Wielkopolska (Chronicle of Greater Poland)
In order to trigger an event you must be the king of Poland, truly follows Christian or Slavic religion, have Polish culture and a Brave trait, when conditions are met, event appears automatically after 1 month.