The search for the Easter egg started off with @noiseminded’s first tweet:
https://twitter.com/noiseminded/status/1432341888774856705
Important notes from noiseminded about the secret achievement:
There is no trial and error involved. I respect your time.
Everyone can do it. No programming skills or browsing the game files is required.
Just wanted to make this clear because I don’t want you guys to go nuts trying every single thing.
So searching through the game files will not let you earn the achievement legitimately, you have to describe what you did to him to correctly solve it. Supposedly learning the games secret language is not part of it, but we’re not 100% certain on that.
In his promo tweets, several phrases are used that also appear in game. They appear in the coded language when collecting the black orb on night 3:
Abstraction says hello.
A sound unheard revels in authority.
Gasoline is a storyteller without equal.
Nothingness Jumps Both Ways
“A Sound Unheard Revels in Authority”
Everything that has been in his twitter related to Nightslink has been in the actual release, so this probably is too.
After this specific tweet was posted in the discord, Noiseminded responded with this image which spells out “STILL INSIDE” in the coded language
More Tweets:
https://twitter.com/noiseminded/status/1337875140412108809 (abstraction says hello)
Second Tweet
Message from Noiseminded about the “Key ahead of Time Tweet”
https://twitter.com/noiseminded/status/1432341888774856705
Hello, Nightslinks. I assure you, I check these threads every day, and your efforts do not go unnoticed. I encourage you to once again read the hintS that I gave on that tweet.
Good luck!
Based on his message there are multiple hints in the tweet, and he clarified that hint(S) meant plural.
Menu Title Hint “GO OUTSIDE”
After the first time you beat the game, any time you restart or open up the game. This screen briefly flashes with “GO OUTSIDE” spelled out in multiple languages, including the games hidden language.
The achievement itself spells out “LOVE” in capital letters
Letter Hints
Noiseminded began to give out hints in the games coded language slowly. The first 2 were images, then a sound file with a hidden letter, and then an “Untitled” game. So far it spells out “RETR”
First Image (1st Hint)
For the first and second images he posted in the discord, the solution was to remove the shadow and turn up the brightness to reveal an “R” and a “E”
Hint 1 Img 
Solved For “R”
Second Image (2nd Hint)
Hint Img 2 
Solved for “E”
The second image has the symbol for an “E”
Sound File (3rd Hint)
He posted a short song clip for the 3rd hint
The sound file has a hidden symbol for a “T” in the spectrum view of the song:
(Not related to the hint) The song is taken from
https://www.youtube.com/watch?v=_xmvHozLjsM and is called the Highland Fairy Lullaby
Untitled Game (4th Hint)
Using the three letters (RET) we solved from the earlier hints, it works as the password to access the game file at:
https://noiseminded.itch.io/untitled
The solution was to listen to the steps. Some parts of the floor have lighter steps than the other. Listening closely, we can spell out the coded symbol which translated to a capital “R”
Extra Info
We overcomplicate it quite a bit. The hints he gave were actually pretty clearly answered, and only took a bit of extra out of the box thinking.
Symbology
