Now that you have freed his family, it is time to find Lagos and attempt to convince him to leave the cult. Ride to Fort Samikon, located south west of the Valley of Olympia in the Forest of Soron. Approach the fort from the western wall and you will gain easy access to the room in which Lagos is waiting, without any guards noticing your entry. When you reach the door, interact with it to triggr a cut scene. After a brief exchange, you will have two options; to kill Lagos immediately, or to convince him to leave the cult and meet with Brasidas.
If you are able to persuade Lagos to leave the cult quietly, he will agree to do so and the quest will end. If not, you will have to kill him in the room. Focus on breaking his shield defence with a series of heavy attacks, and utilise any powerful AoE skills that you have in your arsenal. Do this while evading or blocking any incoming attacks. He does not possess any special attacks and should not be a problem to take down. Once he is dead you will be able to leave the room and return to Myrrine and Brasidas.