As you continue through the story from meeting Hydor. You will travel through "Twallusk Mountain". Once you reach the other side, there will be a small bridge and then a large black monument with floating bricks and some circles around it, named "Apparth Ruins". You won't be able to pass it without it appearing on your screen. Approach it and enter for this trophy to unlock.
Note the door inside is locked, so that is all you can do here at this point in the game.