This page in our guide for Assassin's Creed: Odyssey focuses on the following quest: Battle of Amphipolis.
Battle of Amphipolis
Speak with Brasidas. This conversation depends heavily on your previous choices, i.e. Nikolaos' fate. Escort the man to the battlefield - a few groups of enemies will stand in your way.
Soon, you will participate in the decisive battle. Deimos arrives - this encounter won't end in a fight. Run after Kleon to the beach and deal with him.
You now have to make a choice - you can kill Kleon fast or make him suffer. Rewards: 31,375 XP. This is the end of the chapter - the next, and the last, one is short, too.