Let's move on to the Achievement conditions first:
SMT
The internet died... and i was given life.


Every night of any given day, the game generates any number from 1 to 100, and we need to get the number 66 in order for the achievement to activate!

IMPORTANT: You need to have 100 mental darkness on any given day, otherwise nothing will work.

The Achievement guide itself:

1. Choose any day where you have 100 mental darkness
(for example, 28 day, like mine)

2. We reach the night
(you can just GO OUT with Ame, but not to the hospital, because there the mental darkness will be lowered)

3. If nothing has happened, we re-enter on the same 28th day and reach the night again.
(via the "Load Game" button in the "DAY ##" icon at the bottom right, or in the "Start" button)

4. Do this an indefinite number of times
(I was given an achievement after 3-4 times, it may take someone more attempts)

NOTE: if you already have more than 16 endings, then the chance of this achievement increases!

The following information will be about the Achievement itself, but it will be taken from Wikipedia, as I myself was wondering what the FUN of this achievement was.
As the article says:
- Several aspects of this achievement reference the Shin Megami Tensei series.
The description is a reference to a tagline for Shin Megami Tensei III: Nocturne:
"Tokyo died... and I was given life."
- The achievement title itself is also a reference to the Shin Megami Tensei series;
SMT is an abbreviation of the series's title.
- This achievement's cutscene references an urban legend about the SNES version of Shin Megami Tensei. The rumor in question claims that there is a small chance for the game's screen to fill with red text which reads, "すぐにけせ." This text is usually translated as, "Turn it off," but can also be translated to "Delete immediately," which matches the text shown in the SMT achievement cutscene.
The rumor itself has been debunked.