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 
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, 
Ventricle Razor, or 
Forget Me Now after defeating 
Mom, escaping the boss room. The door can also be opened with 
Broken Padlock, 
Cracked Orb, 
Faded Polaroid, Error: The given pickup ("Soul of Cain") does not exist., or 
Sharp Key, but not 
Mr. ME!, 
Dad's Key,
Mysterious Paper, or a 
Get out of Jail Free Card.
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 in the second floor of the chapter. Destroying it will drop a guaranteed Error: The given pickup ("0 - The Fool") does not exist. (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. The Telepills will become a ??? if Isaac has Little Baggy and 
False PHD, rendering it more difficult to exit the boss room.
 Mother Unlocks
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 The Binding of Isaac: Rebirth
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