Now that you have found Thespis and brought him back to safety, you should find his muse, Aikaterine, who is directly west of your current location at the House of Aphrodite. When you arrive, speak with Aikaterine about why she is unable to attend the play. She will inform you of an Athenian commander who has instructed her that she will pay with her life if she disobeys him. The commander is a member of the cult of Kosmos, and can be found at the barracks north of the House of Aphrodite. Go there and kill him. The barracks are not heavily guarded and the commander has no special attacks. You can ascend the walls and headshot each guard to take them out instantly. Confirm the cultist kill and then return to Aikaterine. You can then decide whether to take up her offer. It will not influence the outcome of the quest. After the cut scene, return to Aristophanes at the Theatre of Dionysos to view the play and complete the quest.