Complex sentence: Jagat predeceased his father and left behind a son Maha Singh--by the Hindu custom of primogeniture, on Man Singh's death this grandson Maha Singh should have been the next Raja of Amber.
Split into simpler sentences: Jagat predeceased his father and left behind a son Maha Singh. By the Hindu custom of primogeniture, on Man Singh's death this grandson Maha Singh should have been the next Raja of Amber.