I've seen several rule systems that give a bonus for magic working on you if the caster knows your "True Name".
It would seem to me that if that was the case everyone would go by a nick names, alias or appellation.
How many players never use their "True Names" and do they pick their alias or do other party members?

Should "True Names" always have a mechanical effect in game?

What about "Royalty"? Should their names be a source of power, for at least political purposes?

How practical would it be to have "True Names" also give strength as well as 'weakness'? 

True names are popular in fiction, where they frequently serve as a plot element that must be uncovered to defeat a particular opponent. Unfortunately, that structure doesn't work well in most RPGs. RPGs tend to have many opponents that are briefly encountered, defeated, and then are gone from the game, and giving them all true names to be uncovered it pretty pointless. Even with recurrent NPCs or boss monsters it's questionable, because mystery-like elements work best when they're not essential to the successful completion of the adventure. For true names, that means the effects have to be fairly minor, which makes them a lot less compelling.

It could work well in a game focused on intrigue or mysteries. Something set in an imperial court where the PCs try to gain leverage on various courtiers, possibly including their true names, for instance. But that's rather different than the standard band of wandering adventurers.

The idea of "True Names", as I understand it, isn't necessarily about legal names or the name given to someone at birth, but more like a metaphysical idea of a special name, based on some type of sacred language, that speaks to the true nature of a person or being. From a mystical point of view it deals with the idea of gaining an insight into the true nature of reality. The idea is that someone who knows someone else's "true name" has some sort of power over them because they understand their true nature or something to that effect. It wouldn't even be relevant unless you make a point of it in your campaign.

I'm not really sure how to handle "True Names" in terms of the game rules cuz I've never tried it before, and I'm not even sure such a concept would work outside of literature, where an author has more control on the course of a story, or myth, where a story is there to teach you something. Within a proper story or myth learning someone's True Name can serve a greater purpose for something specific that the protagonist is trying to accomplish (or is meant/destined to accomplish at some point), but in a collaborative game with many players with potentially conflicting agendas who are just there to "play a game" it can become a source of grievance if one PC learns another PC's or important NPC's "true name" and starts messing with them or sabotaging anything they do.

Unless the PCs are supposed to near the True Name of a demon or evil demigod so they can banish it from their world, I'm not sure how to make True Names work in an RPG. But there's probably some games that have already dealt with the concept (none come to mind now, but I think I read a game where they came up years ago) and might provide some clues on how to gamify True Names.

Nah I'm not running any games where that's an issue. Maybe if I ever did an Earthsea game

Earthdawn dug into this concept.

https://arkanabar.tripod.com/threads.html

Another reminder I missed out on Earth dawn as a youngjn

I actually do true names to be your full legal name with all legal titles. So
Casting a spell on the queen of England that requires a true name is very difficult. While mister Robert Jackson is much easier.

Quote from: Ratman_tf on April 17, 2021, 06:18:55 PM
Earthdawn dug into this concept.

https://arkanabar.tripod.com/threads.html

Ah, that must have been where I saw this concept handled in a game before. I got the core book ages ago (in the 90s), but never got around to playing it. Hence, I forgot. If I ever want to deal with "True Names" I'll have to remember to reread this stuff, cuz Earthdawn had some interesting ideas in the way they handle the metaphysics of it.

Both "Beyond the Wall" and "Through Sunken Lands" have use for True Names but it is mostly for Fae or demons and such.
If you have a Fae background then it can be a weakness.

I would think that if you truly knew yourself, that would also be a strength. Order in Chaos sort of thing.
While you may be weaker to things focused towards you, you would be stronger against things that were not.

Quote from: Arnwolf666 on April 17, 2021, 06:40:08 PM
I actually do true names to be your full legal name with all legal titles. So
Casting a spell on the queen of England that requires a true name is very difficult. While mister Robert Jackson is much easier.

So reality is defined not by physical laws, but by human legal codes? Truenamers are... lawyers?

What do you call 20,000 truenamers at the bottom of the ocean?
A good start.

I think it implies that a True Name is not something given by others but something you discern yourself.

Imagine being doxxed by some pissed off wizard who posts one's 'true name' on every tavern  board in the kingdom.

Quote from: DocJones on April 17, 2021, 07:32:42 PM
Imagine being doxxed by some pissed off wizard who posts one's 'true name' on every tavern  board in the kingdom.

"Knowing" a name and having the power to make use of it should be far different.
Yes, you can be hurtful but it's not literal "sticks & stones". Also, sounds like the beginning of a serious Quest.   

True names featured prominently in the campaign present in the Hellbound: The Blood War boxed set for Planescape.

The Yugoloths plan to exert tighter control over the Tanar'ri and Baatezu, and their main scheme revolves around a creature with a special power. This creature is able to give any being whose True Name is known to it the ability to Teleport Without Error, as per the spell.

The fiends have that ability because this creature is fed each of their True Names. The Yugoloths devise a scheme to dump the creature into the river Styx, which wipes all memory from the swimmer (thus removing from the fiends their ability to TWE at will), and then to teach the creature the True Name of every fiend that is, shall we say malleable to the Yugoloths' demands.

Those fiends that refuse to deal with the Yugoloths never recover their ability to TWE. It's an incredibly major development in the setting, and the PCs can be directly involved with it, even if they are likely unwitting.