This section of our Assassin's Creed Odyssey game guide contains a walkthrough for "An old enemy and a new ally" quest.
After visiting the seat of Cult of Kosmos, go to have a talk with Herodotos. You must visit the Malis region and go to Thermopylae. A short cutscene awaits you, in which you will see Leonidas on the battlefield. Speak with the scholar later and decide that you have to go to Andros Island.
Go on a ship and sail to the island. Another cutscene awaits you there. Go to the mysterious gate and enter it. Reach the spear recess and place Leonidas' spear in the middle. Now you'll come back to the present day - if you want, you can take a break from Ancient Greece and e.g. check your emails, or look for easter eggs in your rooms.
When you're ready, return to the main character and times of Ancient Greece. You'll learn that your spear has been upgraded, which unlocks skill development and new abilities.
From now on, you can constantly improve your spears - you will need the debris that can be found next to the corpses of the followers of the Cosmos.
Go outside and you will meet Deimos. You will have a conversation with the character and, at the same time, another important choice awaits you. It all comes down to the way you speak with him - if you are unpleasant and finally attack Deimos, in the future he will be mistrustful - your relation will deteriorate. However, if you choose the option that you still perceive him as your brother, it will be easier in the future to establish better relations with him and convince him that worship is madness.
After the conversation, go back to your ship and talk to Herodotos. The quest and the whole chapter will end. Reward: +6750 XP.