Assassin's Creed Odyssey

To unlock this achievement you need to discover Atlantis and speak with Pythagoras. During Episode 7 after defeating the leader of Paros island you are directed to an island called Thera to seek someone out. Once you get to this island you will need to solve a simple puzzle in order to open the door that leads to Atlantis, just look for the light source reflecting from the mirrors and destroy the obstacles that block the light, while positioning moveable mirrors where necessary, until eventually the light reachs the top of the door and opens.

Once inside, follow the linear pathway until you meet Pythagoras. The achievement will unlock at the end of the cutscene.