Complex sentence: Nothing is known of his early life before 1259, when he is mentioned for his participation in the events that led up to the Battle of Pelagonia, where he led a contingent of Vlachs.
Split into simpler sentences: Nothing is known of his early life before 1259. John is first mentioned for his participation in the events that led up to the Battle of Pelagonia in 1259, where he led a contingent of Vlachs.