The late 15^th-century tombstone of Mikołaj Myszkowski is one of the most valuable historical memorabilia in the museum. The tombstone is framed by a strip of the Gothic minuscule. In the middle of the slab is a standing figure of the deceased with folded hands, wearing a long robe. This is most likely the chancellor of the Duchy of Oświęcim during the reign of Duke Jan IV. The coat of arms of Jastrzębiec is visible in the bottom part of the slab. The tombstone was initially located in the Holy Cross church of the Dominicans in Oświęcim. Afterwards, its fate was unknown until the World War II when the Germans used it to reinforce the banks of the Soła river. It might have been broken into three parts then. In the early 1950s it was placed next to the tower at the castle hill, attached with steel anchors and mounted on cement mortar connecting all parts. After its maintenance in 1998 it was transferred to the Oświęcim castle.