This door, worked in an elaborate frieze displaying some glorious scene out of history or legend, is in fact a carefully designed trap. Hollow, and balanced carefully on a horizontal pivot while giving the appearance of opening normally, the lower portion of the door contains a liquid; meanwhile, the upper portion contains a powder, kept separate by an easily-dislodged plate. The trap is sprung when the door is disturbed; pivoting on the horizontal axis, the powder and the liquid mix freely, reacting to create a deadly gas that erupts out the myriad hidden holes amid the elaborate frieze, giving the door the appearance of belching the deadly fumes out from countless nostrils, mouths, and eyes.
Generally, this door is used to guard sealed areas which none are meant to enter, or doors which the real owner of the area can bypass by some means, be it a concealed door nearby or magic to skip past it entirely. Particularly paranoid and antisocial individuals might even employ them in the living spaces, although they will tend to reside on a higher floor, safe from the dense fumes.