Kirio, you climb up into the impenetrably dark dragon's mouth, pulling yourself into a smooth stone horizontal shaft. From the perspective of the rest of the party, it seems as if you are slowly being consumed by the beast, your legs dangling and then sliding into the maw at almost ceiling height. You pull yourself along for no more than 3 or 4 feet when your head and arms emerge from the darkness. The shaft opens into a square stone chamber, about 15 feet by 15 feet. Facing you, on the opposite end of the room is a thick stone door. Emblazoned on the surface of the door are large runic characters that glow a bright reddish-orange. The runes provide the only illumination in the chamber.
To actually get into the chamber, you would have to drop down from the shaft (its at the same height as the dragon mouth on the other side of the wall).