Phoibe has been delayed in her return to Aspasia, who thinks that she might be found at Anastasio’s house. Make your way east toward the house which is marked on the map. When you are close enough to the house, use Ikaros to scout the area and identify any important features, including the exact location of where you need to be to progress with the quest. When you reach the marked room within the house, investigate it by interacting with any items that have a search icon associated with them. When you are done, walk out the room and loot the chest to your right. You can then proceed outside and the quest will update.
You must now run to Phoibe and protect her from the cultists at the Odeon of Perikles. When you enter the odeon you will find four cultists armed with shields. Focus on striking them with heavy attacks to weaken their defences, and strike swiftly with light attacks or special attacks when they are vulnerable. Avoid straying too close to them as they hone in on you with multiple attacks. When the last guard falls, a profoundly sad cut scene will follow, in which Phoibe succumbs to her wounds. When the cut scene ends, the quest will complete.