Complex sentence: Peliocles and his men are captured and forced to provide amusement in the local arena; but just when Darios arrives to publicly expose the traitor's plot, Thar executes his coup and kills Serse and his retainers.
Split into simpler sentences: Peliocles and his men are captured and forced to provide amusement in the local arena. But just then Thar executes his coup and kills Serse and his retainers.