This achievement is unlocked when you complete the Emil's Memories
side quest. In this side quest, you acquire the key to the elevator in the commercial facility from Emil. Use the key to descend the elevator. Emil will show up and say a few things. When he finishes talking, you will get the notification that Emil's Memories is complete and the achievement will unlock.
--During the Forest Kingdom Main Story Mission (Chapter 6 if you are doing this later in the game with Chapter select), you encounter Emil for the first time in the Commercial Facility as you make your way to the Forest Kingdom. When you finish the Forest Kingdom main story mission (and begin Chapter 7), Emil will operate a roving shop. It moves quickly and you have to attack it to stop it using ranged (recommended) or melee attacks. Do so and interact with him. Consult this guide if you need more detail surrounding how you initiate this first contact which also unlocks an achievement: Solution for Come Take a Look! in NieR: Automata BECOME AS GODS Edition
--Go back to the Commercial Facility and search around the area where you first saw Emil. You will find a glowing white flower. This is a Lunar Tear. Emil will show up. He says the flower brings back memories and to notify him if you find more. This initiates the Emil's Memories side quest. There are four more Lunar Tears to find.
--Markers in the form of red circles now appear on your map identifying the locations of the four Lunar Tears. You can find them in the Flooded City, the Amusement Park, the Desert, and the Forest. Three of the four are in plain view but the one in the desert requires first going into the subterranean caverns and then executing a slightly tricky jump so that you can access a recess behind an elevated area.
--Each time you find a Lunar Tear, Emil is notified automatically and shows up almost immediately. When you find the fifth Lunar Tear, Emil gives you the key to an elevator in the commercial facility. He does not directly state it but gives a hint that the key also operates an elevator leading to the subterranean caverns where he lives. Before you use the elevator, make a save. When you go down the elevator in the commercial facility, Emil will show up and say a few things. This concludes the Emil's Memories side quest and you will get a notification to this effect. The achievement unlocks along with this notification.
If the achievement does not unlock before you go back up the elevator, you most likely have experienced a glitch. In the event of a glitch, clear the cache. To do so, turn off the console by holding the
button on the console for at least ten seconds and then unplug the console for at least 20 seconds. Load the save you made before going down the commercial facility elevator. Clearing the cache will most likely remedy the problem and the achievement should unlock when you get the notification that the side quest is complete.