Eternity box
Still around the background music from 999, the "Eternitybox" one gave me a strange vibe. It's a relaxing but uncomfortable song. It's literally the song for a moment you enter an unknown room and start looking around everywhere to make sure you are safe, or where you investigate to understand how can you escape from there.
What I didn't know was the meaning of the word. Eternity box is a way to call a coffin. And it makes sense, since it is where the body stays for the infinity.
This meaning may be related to Snake and the Coffin ending. Snake is alive inside that coffin, but without the password, he will stay there for the eternity. Because whoever is trapped inside one, can't leave (that's also a recurrent theme in thriller/suspense movies and series, and to be honest it feels quite scary to be trapped inside one).
On the other hand, the whole spot where the Nonary game is being held is itself an eternity box. Each room is an eternity box. And besides being places where they have to seek a way out, they also almost work as Schrödinger's cat box. Anyone outside doesn't know if whoever is in the room or the building is dead or alive. And that's also the mystery tone the background music gives.
In the end, every single challenge in 999 is made of eternity boxes that they are supposed to open, finishing with the coffin that brings up the Light, and the tinderbox (the incinerator, also a track from the game bgm) where everything is cleared up and we finally get to see if Akane was saved or not.