After the liberation of Kenabres and the Gray Garrison, you will find yourself on a huge map of the world and will be able to travel both as a detachment and as an army. Not far from the starting point will be the location The essence of the unknown. Go there and find a cave. There is a mystery inside. Search the urns and collect the slabs. Place ordinary plates on the sides. There are three of them. And double slabs need to be placed so that they are combined according to the domino principle.
Here’s the answer: