Complex sentence: Aqua and Mickey fight with the possessed Ventus and destroy the χ-blade, while Ventus engages and defeats Vanitas in a metaphysical battle within his own mind, losing his heart and falling into a deep sleep as a result.
Split into simpler sentences: Aqua and Mickey fight with the possessed Ventus, while Ventus engages Vanitas in a metaphysical battle within his own mind. Aqua destroys the χ-blade as Ventus defeats Vanitas, though Ventus loses his heart and falls into a deep sleep as a result.