You need to have passed through the tunnel where you fought the huge robot. There's a tunnel leading out past it. From there, hug the left-hand side until you find a path down the beach, From there keep going southwest, back towards Cyseal, until you reach a place with a rock arch and a bench. If you have the appropriate quest - you've read Evelyn's log - your characters should prompt you that this is the right place with a conversation. From there, right-click the Reveal spell in your inventory.
This might be the wrong place, but I do have a moment of praise for Larian here: On the second time I visited that place, I knew where the entrance is, so I wanted to see if the entrance was non-existent and comes into being when the spell is used. To that end, I tried to make a character embedded into the entrance by walking them to inside where it should appear, but when I tried, I couldn't get close enough. The entrance, being invisible instead of non-existent, correctly blocked me from approaching the wall.
