You’re going to come across a great deal of optional content in Divinity: Original Sin, and this puzzle is just a taste of what’s to come brainteaser-wise.
Some players may only want gentle nudges in the right direction. Tiers of spoilers (“if you do X, X will happen”) are separated by (—). If you feel you have read enough on how to progress and don’t want what’s further to be spoiled on the solution, stop at the (—) marker.
You come across a hatch next to a dilapidated home to the west of Cyseal, just south east of the area with fire elementals. Found in the hatch is a cave adorned with with four elemental pillars and a tome which reads:
Water
Earth
Air
Fire
In this cell’s double, your double will face your ally.
Cryptic, right? The key word to keep in mind from this, aside from the list of elements, is “ally” — meaning a party member (or two) as opposed to the entire party. This should tell you to split your party when you find the other cell. (Note: This cell is called the “first room” further down the guide.)
The other cell is just north west of this one, in the area with fire elementals. Fight them and make your way down the hatch to find the room mentioned in the tome.
This room has similar elemental pillars to the other, but the words on the tome itself are a little different (Note: This is called the “second room” further down the guide.):
Earth
Water
Air
Fire
In this cell’s double, your double will face your ally.
Notice the elements are in a different order, and again it mentions “ally”, hinting you should split your party. Do just that, and send two party members to the first room.
(—)
Figuring out the order to pull the levers
The first cell you came across asked for Water, Earth, Air, and Fire. The second cell requests Earth, Water, Air, and Fire. As pulling a lever sets off an elemental attack in the opposite room, you may assume you need to pull the levers in the order the tome in the opposite room requests — and that assumption would be correct.
(—)
Before pulling levers willy-nilly, be sure to move your character(s) in the opposite room to the outside of the middle of the pillars. The pillars’ attacks aim in the center, and they do hurt and apply status effects.
Order of levers in the first room
You must pull the levers in the first room in the order the tome found in the second room lists: Earth, Water, Air, and Fire.
Order of levers in the second room
You must pull the levers in the second room in the order the tome found in the first room lists: Water, Earth, Air, Fire.
(—)
After you’re done pushing buttons and pulling levers
A mysterious enemy will appear in the opposite room once you have finished a lever puzzle. This happens in both rooms upon puzzle competion, each fight against one level 5 opponent modeled after one of the default characters.
After these fights, be sure to pick up the pendants and loot that drop to the floor.
Head down the newly-revealed hatches in each room to come upon two pedestals, one on each side. Drag the pendants from your inventory to their respective pedestals to open the gates and receive a good deal of loot for lower level characters.
