BELOW

As I've recently completed the game, I wanted to provide a more fleshed out guide for this achievement.

The important stuff:

This is the last achievement you will unlock. You MUST collect all 20 Lantern fragments (it is required for the story). In doing so, you will unlock multiple other achievements along the way (1 Bit, First Light, Depart, Last Light, and of course Dark Night. At least, that is the order I unlocked them in).

Once you have obtained the 20 fragments, you first want to go back to the Lighthouse. The Lighthouse is located in the starting area on the upper part of the cliff on the far left hand side. You can access it after unlocking the Level 10 shortcut. Head inside and press to activate the lift in the middle of the room. It will take you up top and outside. Press again, and some stars in the sky will light up. Wait as the cutscene plays out, and once you are in control again, head back down. If you had never previously gone up to the top, after you collect the 20th fragment, you would see a "vision" showing the top of it in black and white.

From here, you need to head back down to Level 16. You'll need to find the center room, and use your Lantern to open the door(s). Once you are inside, some more stars / lights will light up, and then a door will open. This is when you will unlock Dark Night, as outlined in its solution. Next, walk up to the monolith. Because you went to the Lighthouse first, you should lose control of your character and the end-game cutscene should begin. You MUST go to the Lighthouse first, as if you come straight to Level 16 and walk up to the monolith, nothing will happen. After the cutscene finishes playing, the achievement will unlock.

Some extra points / tips:

First, I recommend you play on Explore Mode. Explore Mode makes the game much easier to play, and ultimately complete. I made decent progress in Survival Mode, and started fresh with Explore Mode and I’m really glad I did. I got to the same point I was at in Survival Mode rather quickly, and progressed through the game much quicker as well. If you want the extra challenge, that’s up to you.

Shortcuts are very helpful for progressing within the game. They can be found on Levels 2, 4, 6, 10, 12, and 20, with 20 being the final floor. For endgame setup, I would HIGHLY recommend setting your final campfire teleport to the campfire in between levels 16 & 17. This way, you only have to go up 2 rooms to get to the center room to finish the game. Unlike previous levels, you cannot go back up to the previous one starting at Level 12. This is also a pretty decent spot in case you forget something / die / etc., and need to make your way down through Levels 17 & 18 and back to Level 20 again. In my experience, for both Levels 17 & 18, you just need to go straight down twice to exit the level. Even after I died, this was still true.

With crafting, I found what helped me was to prioritize materials for bandages and bombs specifically. Bombs will be needed to clear rubble for some Lantern fragments anyways, and bandages are extremely helpful to have for Levels 14-18 with the Darkness. Check out the Wiki here: https://below.fandom.com/wiki/Crafting?so=search

If you should die, spend time doing a resource farming run. My approach was once I started unlocking the shortcuts from the starting area, I would quickly go to each of them and open the chests in the immediate area, or ones that I knew were close by. Once I did that, I then quickly went through Levels 1 & 2 to gather the light orbs that power the lantern. Once I was stocked up, I headed to the Pocket and deposited everything. Then, I would go to any level and let the traps / enemies kill me. Then on the next run, I would do the farming run quickly again, AND go grab my things from the Pocket. At this point, I would have lots of bandages and decent amount of bombs in case. Then I would use the nearest campfire and travel to my previous teleport point and proceed.

And finally, even though there is no achievement for it, I would still highly recommend collecting a few of, or even all of, the marbles to upgrade the Pocket. The extra storage space really comes in handy.