For this quest you must find the ruthless pirate, Xenia, who can be found on the island of Keos. The island is located south east of Athens. Sail to the north of the island and dock your ship. Make your way to the Temple of Athena Nedousia and use Ikaros to scout the area. You can then move toward the entrance of the temple and interact with the door to initiate a cut scene, followed by a dialogue with Xenia.
She will inform you that information will be given to you, but only for the right price; 15,000 drachmae. If you do not currently have this amount of money, you can focus on side quests or aim to complete other main quests first. If you are able to pay now, do so. You will be able to loot a number of chests in the following main quests which will reimburse much of what you have given. When you hand the money over a cut scene will initiate, after which you can question Xenia and bid her farewell, completing the quest.