A Strange Door
A Strange Door is a special exit that is unlocked by beating Mother for the first time. It leads to a special version of Mausoleum/ Gehenna II, which contains Dad's Note, which starts the Ascent, and eventually bringing you to Home and the fight against Dogma and The Beast.
The door spawns in the starting room of Depths/ Necropolis/ Dank Depths II. It can be opened by giving it The Polaroid or The Negative. This can be done through methods such as using a teleportation card, a Telepills, a teleport activated item, or Forget Me Now after defeating Mom, escaping the boss room. The door can also be opened with Broken Padlock, Cracked Orb, Faded Polaroid or Sharp Key, but not Dad's Key or Mysterious Paper.
Having more than one of The Polaroid or The Negative, (such as through Diplopia or R Key) will only remove one to open the door. Faded Polaroid is always taken first if held, then The Negative and then The Polaroid.
A Marked Skull always spawns somewhere on the chapter. Destroying it will drop a guaranteed 0 - The Fool (or a Telepills if holding Little Baggy), giving a reliable way to open the door. It is recommended to visit the Boss Rush room for a free item before teleporting out if possible.
Note that defeating Hush thrice counts towards unlocking A Secret Exit, allowing the player to unlock Corpse without unlocking the Polaroid or Negative. This makes the Faded Polaroid, Sharp Key, or Broken Padlock the only ways to open the door until these items are unlocked.