If you are talking about after using the mirrors in halls of insanity, it is like a save point, with a replay boss message on it (some demon expression), you find it at the very top of the halls of insanity, pressing spacebar sends you to another location where there is a mirror and spikes surrounding that mirror area, so call it a trap, that may be what the ??? is.
Whether you can actually get out of it isn't clear, but if you have invincibility and flaming sword you can risk going to use it to get out safely by that way, or before use use shield that provides timed invincibility and then activate the ???.