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Q: You can control a Demon by knowing its True Name, but why?

Serban TanasaThe power of True Names over demons is unbelievable: A Lamashtu demon can drain you of your blood in under 5 seconds, blend into shadows, and travel at the speed of darkness. Yet it will crumple like a wet napkin, if only you know its True Name. Your average Glabrezu is 18 feet tall, 4000 pound...

Isn't the answer to this trivial: because that's the way you wrote the rules for demons in your universe? Or are you looking for an in-universe explanation?
In-universe, plausible explanation. How would such a situation come about? Demons are as smart as humans, perhaps even smart as AIs. Why would they just have such a vulnerability?
Demons are limited (bound) by contracts + Contracts are sealed by names (signatures) = Demons bound by names. Yeap, that's D&D alright. You might wanna give us some details on what you're looking for (scientific explanation/magic binding etc). I see that you're no novice :-) Otherwise we can look it up in D&D manuals.
If the demon kills you once you know its name, you can tattle on them to their mom in the next life.
You say you want an answer involving as little magic as possible, but the answer is actually "magic". There isn't a scientific explanation, it's plain and simple magic, the nature of these demons is simply that their true names grant power over them.
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Wait. You're talking about demons but you don't want to talk about magic? Huh? What?
Now, big dragon, little wizard, take your true shape. I command you by the power of your true name-Yevaud" Birt could not move at all, not even to blink. He cowered, staring whether he would or -it. He saw the black dragon hang there in the air above Blackbeard. He saw the fire lick like many tongues from the scaly mouth, the steam jet from the red nostrils. He saw Blackbeard's face grow white, white as chalk, and the beard-fringed lips trembling. "Your name is Yevaud!" "Yes," said a great, husky, hissing voice. "My truename is Yevaud, and my true shape is this shape."
Satan and demons are all about lies, so maybe they derive their power from deceit. Using their true name illuminates their core truth and dissolves their power.
Maybe you know the True Name of the demon's boss.
@WhatRoughBeast While I cannot guess what Serban has planned, I have explored situations where you might want to talk about something fantastical like demons while avoiding magic. Sometimes its fun to put together a fantastic, potentially scary, story with demons or wizzards, but leave the reader pondering "might this be possible in real life? Could this actually work?" It encourages the reader to pay more attention to the morals in the story.
Note that in some universes, the same is true for humans, not just demons - this at least makes it a bit more symmetric. This is tied to the old (and kicking) belief that names have inherent power. @WhatRoughBeast Magic isn't an explanation - it's just naming the confusion. The true nature of True Names may not be completely understood, but investigating the issue may still be scientific (and often is, at least in part) - you learn as much as you can about the principles, which allows you to predict future behaviour based on known facts.
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The answer to most fantasy questions: because your in a simulation that allows beings to be bound by arbitrary rules.
There's the Anthropic Principle. If demons were that powerful and didn't have any weakness, there would be no non-demons left alive to ask about the nature of demons. That doesn't explain how the weakness works, but it does in a way explain why one exists.
@Frostfyre "Because that's the rule" - Nice answer for most of the questions here. Shortcuts actual discussion nicely :P
Were you inspired by the Bartimeus Trilogy? I see some interesting similarities... Yours is a great question!
I was thinking it would break the dissociative state that the demon has to be able to function in to be as evil as they are. Before the demon's name is said it can pretend that they are not responsible... but when you say its name you snap it back to a reality where it is once of the most evil creatures in existence.
I don't see how this is related to world building. Perhaps you meant to post this question on rpg.stackexchange.com ?
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Because their real name is Maryanne and they'll do anything to stop you from telling anyone.

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