This location is in The Mourning Fields. This is one of the most important locations in Assassin's Creed: Odyssey Fate of Atlantis - you reach it during the main storyline.
Enter the cave and go right. Before you move forward, you can collect raw materials lying on the rocks.
An altar is at the end of the tunnel. There are many different resources on it. After collecting them, go left.
Achilles is standing on your way to the tablet. Kill him - this is part of another mission.
Go up the stairs to find the tablet. When you interact with it, you will end this location and one out of the three parts of the quest.