So i was talking with some friends about different topics of lore jumping from place to place, and someone tried to mention something about the black doors (this was actually a day or two ago so I dont remember specifics) but ultimately it ended up becoming a conversation of how they work. I dont know if we have any solid proof or not of how it works, but i created a few, very loose theories about it that I wanted to check with people here to see if theres either a better explanation or if my thought process is on point.

So first off, what we do know about the doors is that they guard every dark brotherhood sanctuary. They are sentient? to the point where they can recognize somebody in front of it, voice a question, and if answered correctly will allow that person entrance into said sanctuary. They also arent all the same. Each sanctuary has a different passphrase to get in and if memory serves, all are known by the night mothers keeper.

What we DONT know is how exactly these doors work, if there are similar structures in the elder scrolls universe that function similarly, and how do those work, or really what the doors are.

I have a few theories myself,

The first and least likely to me is that the door itself is infact sentient. Whether through some twisted soul-gem infused enchantment or some form of magic that binded a soul to the door, the door very well could be aware. This is not my favored theory because the doors seem to have one purpose; What is the password? if you don't get it right you don't get in.

The second theory is that they're binded with some form of illusion magic. We do know that illusion magic has the ability to manipulate sound, as seen through spells like muffle. So its not too far out there to expect these doors to be built during construction of these sanctuaries and have the a spell placed on the doors. With my current knowledge this is definitely possible.

The third theory is https://www.youtube.com/watch?v=lfmfjfLlKB0&feature=youtu.be&t=1m32s

and finally, the theory I think is most likely, the night mother/Mephala has influence on these doors. We already know daedric princes have the power to project their voices through their own shrines to those they deem worthy, I dont think its too far of a stretch to think that they could project their voice from a door to members of the brotherhood. I think this is something mephala, already being the night mother, is definitely willing to do that.

So im curious as to how my theories hold up to people who may be more versed in lore. I'm pretty familiar with alot of lore but when I get into specifics I always feel like im missing key pieces of knowledge so I'm curious how my theories, especially the last one actually hold up!

First is most likely, IMO.

In Morrowind, there was a quest to reach a specific shrine. To reach the shrine, you needed to answer three riddles correctly to three different Atronarchs. Granted, they attacked you if you answered incorrectly, but the point was that they were bound to that specific duty, and only performed said duty. Answer correctly and pass unharmed, answer incorrectly or try to pass them and get attacked. Another quest required you to act out one of Vivec's actions to a (seemingly rather bored) bound Dremora. Guy offers a silver sword, Dremora says a few lines and accepts said sword. That's the only thing the Dremora did.

It wouldn't be a stretch for something similar being put on a door. Seems to me it's lot simpler to bind some poor soul to the door with a single purpose than to jury-rig a bunch of other spells from a variety of schools to do the same thing (it'd be from illusion, alteration, and mysticism, one to make the sound, another to lock and unlock, and a third to be able to figure out when to fire off the other two spells. And it still likely needs a soulgem for the intial enchantment process too.) It also fits with the DB's overall theme- they'd probably bind some random shmuck's soul to the door, or somehow receive a few souls sent to Sithis/Night Mother/Whatever when they're making a new door.

As for the 4th, all daedric statues are shrines and objects of worship. They serve as links to Mundus from their Prince, and act as the mouthpeice whenever a Daedra wants to communicate something to their followers. The doors don't really make sense in that function unless the Night Mother/Sithis operates on an entirely different ruleset than the Daedric Princes. The doors aren't objects of worship, and all communication to the DB from the Powers That Be is through the dead body or resting place of the Night Mother. Seems awefully trite that the Night Mother would sit there and personally interrogate everyone who approached one of the doors, especially considering that she could immediately know who was and was not part of the DB in the first place, and even possibly know who is a new recruit.