This mystery can be found near the centre of Essexe, just southeast of Colcestre. You’ll find the entrance to the cave as you approach it from the north.
Where to Find the Mysterious Horn Tablet
Once you’re inside the cave, turn east to find a hole leading to a series of chambers. This is a fairly linear path to follow, but when you reach the first large chamber you should stick to the right hand side (southeast) as you follow the ledge around it. Continue northeast on the right hand side as you climb upwards, then turn north when a new chamber opens up and follow the ledge on the left hand side. In the next chamber (facing northeast you’ll find a small pool and a number of stalagmites. Keep to the right and follow the path up to the cellar.
Once you’re in the cellar, turn east to find an inscription on the wall, which provides a clue related to solving the puzzle in the cellar. Essentially, all you need to do is shoot enough wine barrels until Eivor comments that they can now reach the tablet. In particular, you’ll need to shoot the links highlighted in red on each barrel. Once Eivor comments “The wine has filled the shaft, bearing my prize with it. I should be able to reach it now.”, you can go to the centre of the room and collect the Mysterious Horn Tablet from the shaft.
Once you have the tablet, exit through the door to the northeast, but be sure to loot the treasure chests before you leave.