This is the end of Alexios' and Kassandra's journey. This page in our guide for Assassin's Creed: Odyssey focuses on the following quests: Where it All Began and Dinner in Sparta (the game's epilogue).
Where it All Began
Go to Myrrine and tell her about everything. Reach the top of the mountain, the one that has divided your family. You meet Alexios/Kassandra. Time for a conversation - by making "good" choices you are able to convince Deimos to leave the past behind. Choose the option where you don't want to fight - this choice will lead to the good ending.
Making a few bad choices and being hostile towards Deimos leads to a fight. Your enemy has the same set of moves as before. Also, Deimos will probably be a few levels below yours - this will make this fight much easier.
The quest ends after the fight. Rewards: 31,375 XP.
Dinner in Sparta
The last mission serves as the game's epilogue - visit your family home. Depending on your choice you can meet with: Nikolaos, Myrrine, Deimos, and Stentor. Make all the good choices and the entire family will reunite. The only person you can meet after taking a path of revenge is Myrrine.
You have completed the main storyline - you can now focus on the newly unlocked Sokrates' side quest. The goal of that mission is to commemorate the fallen and discuss your personal odyssey. Remember that you can go back to hunting the Cultists of Cosmos and complete missions with the artifacts.