Markos's questline starts with the "A Business Opportunity" side quest on Kos Island ("Highlands Of Asklepiades" sub-region). It begins by finding Markos in a cage inside the enemy fort, where the quest marker is displayed. You must free him and carry him back to his house to continue the questline. At the end of the questline, he will give you a nice axe and you will get the "Everybody Benefits" trophy.