The location can be found at the border between Asylum and Deukalion's Heritage. This is one of the more important areas in Assassin's Creed: Odyssey Fate of Atlantis, as you'll visit it while completing story missions.
To get access to the inside of this area, you need to dive and swim up to a cave.
When you reach the end of the underwater cave, swim up to the Surface. This way, you'll find yourself inside the Jaw of the Styx.
To continue, you need to get through a hole in the wall - it is shown in the screenshot above.
Use the stones to get to the other side of the cave. You can also swim, but this will prolong the process.
Turn left at the first opportunity. The next part of the cave will be accessible thanks to a narrow fissure.
Travel to the end of the area - you'll find the table there. Once you activate it, you'll clear the area and complete one of the four parts of the quest.