Complex sentence: In 1293 Andrew III invited his mother to Hungary, who carried out successful negotiations with several rebellious barons (Henrik Kőszegi, Stefan Dragutin), who accepted her son's rule.
Split into simpler sentences: In 1293 Andrew III invited his mother to Hungary. She successfully negotiated with several rebellious barons (Henrik Kőszegi, Stefan Dragutin), who accepted her son's rule.