On this page of Assassin's Creed Odyssey game guide, you'll find a description of how to reach The Tomb of the First Pythia and find an Ancient Stela hidden inside.
Tomb of the First Pythia
Tomb Location: Parnas, near the Temple of Apollo.
Solution: Go downstairs and run in front of you. Turn right, destroy the vases and go through the gap further. Kill the cobra and follow the stairs further down. Kill another snake in front of the chest and open it.
On the wall next to you will find another hole - kill the snake and enter it. You will reach the next chest defended by a snake. Take the treasure and return to the previous room. Go further on and kill the snake. Then turn left and kill a few cobras.
You will reach the cracked wall. Destroy it and you will reach the Ancient Stela. Kill the snakes and run up the stairs.
You'll reach a cabinet that you can move. Unlock the exit shortcut - return to the surface through familiar corridors.