Complex sentence: Thebes, the capital of the Duchy, was abandoned by many of its inhabitants, who fled to the Venetian stronghold of Negroponte, and was plundered by the Catalan troops; and Athens was surrendered to the victors by Walter's widow, Joanna of Châtillon.
Split into simpler sentences: Thebes, the capital of the Duchy, was abandoned by many of its inhabitants, who fled to the Venetian stronghold of Negroponte, and was plundered by the Catalan troops. Finally, Athens was surrendered to the victors by Walter's widow, Joanna of Châtillon.