Complex sentence: Thereupon they handed over Severyn Nalivaiko to the Poles in exchange for their own lives, only to be treacherously slaughtered by the Poles immediatedly after Nalivaiko's surrender.
Split into simpler sentences: Thereupon they handed over Severyn Nalivaiko to the Poles in exchange for their own lives. They were, however, treacherously slaughtered by the Poles immediatedly after Nalivaiko's surrender.