The description obviously speaks for itself. In order to unlock this achievement you simply have to complete the final chapter of ACT III.
After solving the Lighthouse puzzle and shining the beam of light on the stage (bottom right area), a cutscene appears. Shortly after, you’ll need to follow Didi behind the stage to trigger yet another cutscene. The achievement will unlock before the credits roll onto the screen.
Here's a video walkthrough by "GamerrZOMBIE" (credits and thanks to him) if you have trouble solving the puzzles on your way to the top: