Complex sentence: Callisthenes of Olynthus (in Greek; c.360-328 BC) was a Greek historian, a great nephew and pupil of Aristotle, through whose recommendation he was appointed to attend Alexander the Great in his Asiatic expedition as a professional historian.
Split into simpler sentences: Callisthenes of Olynthus (in Greek; c.360-328 BC) was a Greek historian. Through his uncle's influence was later appointed to attend Alexander the Great in his Asiatic expedition as a professional historian.