This location is in the western part of the underworld. This is one of the most important locations in Assassin's Creed: Odyssey Fate of Atlantis - you reach it during the main storyline.
To get inside this place, you need to reach the top of the rock.
Go inside and follow the walls. This location is quite dark. However, if you move along one of the walls, you won't get lost.
To go further you need to go into the hole in the wall - see the picture above.
The tablet is guarded by three dogs. They will attack you as soon as you enter the room. Get rid of them or you won't complete the quest.
Reach the end of the location to get the tablet. When you interact with it, you will end this location and one out of the three parts of the quest.