Complex sentence: Boudica's husband Prasutagus was ruler of the Iceni tribe, who had ruled as a nominally independent ally of Rome, and had left his kingdom jointly to his daughters and the Roman Emperor in his will.
Split into simpler sentences: Boudica's husband Prasutagus was ruler of the Iceni tribe. He ruled as a nominally independent ally of Rome and left his kingdom jointly to his daughters and the Roman emperor in his will.