Having said goobye to Phoibe, you must now seek out Aspasia at the Temple of Athena, which can be found directly north of the Odeon of Perikles. When you reach the entrance you will initiate a dialogue with Aspasia. You can choose to blame her for the death of Phoibe, or simply inform her that she died at the hands of the cult. Either way, the following cut scene will end in the death of Perikles.
When you have witnessed your sibling murder Perikles, you will have to fight two more cultist guards before you can flee Athens with Aspasia, Hippokrates and Sokrates. When the cut scene ends, run with them toward your ship. As you approach the ship five Athenian soldiers will attempt to stop you. They are not particularly tough, just focus on the light soldiers first while avoiding those with heavier armour. When they are all dead Hippokrates and Sokrates will bid you farewell, and the quest will complete.