Assassin's Creed Odyssey: Judgment of Atlantis begins with The Last Call mission. The quest involves getting to Atlantis.
The entrance to Atlantis is located on Thera island. It is one of the Volcanic Islands located in the eastern part of the world.
To get to the gates of the ancient city, you need to go to the Gateway to the Lost City. This will automatically start the mission.
After entering the Gateway to the Lost City, you will have another meeting with Aletheia. She gives you valuable information and opens the entrance to the last ancient land.
Your adventure will begin in the center of Hades' land. Poseidon appears - he will take you to the lost city.
After a conversation with Poseidon, climb to the top of the building. There, you will find a tablet. After receiving the information, go back to Poseidon. This will complete the mission.