Original:  On September 18th Antonys troops entered the capital and captured Artavasdes II . He was then brought to Alexandria and was kept imprisoned by Cleoptatra till he was executed in 31 BCE.
Revised:  On September 18th Antonys troops entered the capital and captured Artavasdes II , who was then brought to Alexandria and was kept imprisoned by Cleoptatra till he was executed in 31 BCE.
Edit intent: coherence