For this quest you must aid the famous sculptor Phidias in his escape from Athens. Meet him at his workshop in the west of the city. As you approach the entrance to the workshop a guard will stop you from entering. You can choose to bribe him or kill him immediately. Lying will have no influence on him. Note that if you do bribe him he will attempt to kill you a little later anyway. You can therefore save yourself the trouble and kill him at the door. Another guard will attack you at this point, but they will not prove to be difficult. When they are dealt with, go inside and speak with Phidias.
After working through the dialogue options a guard will enter the workshop. Kill him and then meet Phidias at the Port of Piraeus, to the west of the city.
Note: You will have to meet Phidias at the port when it is night time. Use the meditation feature to advance time if necessary.
When night falls, speak with Phidias. After a brief exchange you will be transported to your ship. You must now sail to the island of Seriphos. However, before you can properly set sail you will be attacked by three to four Athenian ships. This will be a tough fight if you do not take care to employ some basic tactics. The most important thing to remember is to brace whenever an enemy ship is firing at you. This will significantly reduce the amount of damage that your ship will sustain from arrows and rammings.
As you enter the battle you can attempt to ram one of the ships to gain some initial damage, but if they fire on you collectively you should abandon the ramming and brace before their incoming arrows hit. You should be particularly cautious of fire arrows, as these will inflict a lot of damage in a short amount of time. Maintain mobility as you encircle your target and inflict as much damage as possible from the side with arrows. You can slowly take out each ship in this way.
When you have dealt with the Athenian war ships, continue on your way to the island of Seriphos. When you arrive, dock your ship and the quest will complete after a brief cut scene.