This page of the Baldurs Gate 3 game guide describes the solution to the puzzle with stone slabs with runes. You can solve this puzzle in one of the chambers of the Emerald Grove. Solving the puzzle leads to unlocking access to the hidden vault.
For the record, Emerald Grove is one of the locations within Druid Grove. You visit this location primarily as part of the Removing the Parasite main quest.
While completing Removing the Parasite quest, you must meet Nettie. No matter how you behave when meeting her, you will gain access to the Enclave Library. This is where you will solve the main part of the puzzle. There is a stone statue and four pedestals with runes.
You can't solve the puzzle right away, because you have to get one missing rune first. Go back to the previous chamber where the cutscene with a captive girl and a snake took place earlier. Inside, you will find Rath. The missing Rune of the Wolf is in his inventory.
The rune can be obtained in two ways.:
- It can be pickpocketed. This is risky, so be sure to save the game before that. Moreover, wait with stealing the rune untile the large wolf walking around the chamber heads in the other direction. This will reduce the risk of detection during theft. You have to pass a skillcheck to successfully pickpocket the rune.
- Attack the people in this chamber. You'd have to fight a tough battle with Kagha and other Druids. If you have murdered the occupants of the Emerald Grove on one of your previous visits, then just find Rath's corpse and take the rune.
Return with the rune to the Enclave Library. Interact with the "empty" pedestal in the image above.
The combine items window will pop up. Open the inventory and move the Rune of the Wolf to the empty slot to the right. Then click on the Combine button.
The pedestal with the Rune of the Wolf is ready. You can click on it to activate it.
Do the same to the remaining three pedestals with the rune of the Eagle, the rune of the Bear, and the rune of the Elk.
With the completion of the riddle, the central statue is lowered and the descent is revealed. You can use it.
The reward for completing the puzzle is the ability to plunder the Hidden Vault. Most importantly, you will find there:
- A rare (blue) two-handed weapon - Sorrow. It adds a unique Ensnaring Strike action that summons spiky vines, inflicts slashing damage, and can immobilize a target.
- Rare (blue) Robe of Summer. It provides resistance to Cold Damage.
- Various high-quality potions, poisons, and scrolls. Look inside the crates that are not marked when you hold down the ALT key.