1 Iki Piki: I can feel an opening here.
1 Quercus: Wait. This reminds me of something.
2 Quercus: The demon mouth in the old Dungeons & Dragons adventure, Tomb of Horrors.
3 Quercus: An utterly black portal. There was no way to know what would happen to anyone who went through.
4 Serron: Well I’m sure it turned out fine. Let’s go! {steps into the portal and half his body vanishes through the curtain of blackness...}

The demon mouth is possibly the most infamous thing in the adventure module Tomb of Horrors (which I've mentioned before). I won't link directly to an image here, but if you do a Google Images search you can find several images of the demon mouth in question.

It's a greenish demon face, a bit taller than a person, mounted on a dungeon wall, with a gaping mouth large enough to step through if you squat down. It's impossible to see anything inside or beyond the mouth, as it appears as a wall of absolute blackness, though there is nothing tangible there. You can toss objects through the mouth, or even feel inside with a hand, and it just feels like empty space on the other side. There's no way to figure out what will happen if you step into the mouth. And it's very possible at this point of exploring that there are no other obvious ways to progress any further into the dungeon.

I won't spoil exactly what happens if you go through the demon mouth. Let it be enough that when I played Tomb of Horrors for the first time (my brother was acting as the Dungeon Master), our party decided to go ahead and entered the mouth.

It turned out fine.