True name
From The Inheritance Encyclopedia
"Those who speak the Ancient Language have two names. the first is for everyday use and has little authority. But the second is their true name and is shared with only a few trusted people.There was a time when no one concealed his true name, but this age isn't as kind. Whoever knows your true name gains enormous power over you. It's like putting your life in another persons hands. Everyone has a hidden name but few know what it is." Brom - Eragon pg. 146.
"A true name is a word or group of words in the ancient language that is the essence of an object, person, or thing. Everything in Alagaësia has a true name, and knowing someone's true name gives a person complete control over the object, person, or thing. Knowing the true name of a person gives immense power over them and can force them to do your bidding." Brom tells this to Eragon when they are traveling together through the empire in Eragon.
"A true name can be of great importance magically, but it is not a spell in and of itself...If I gave it, you might profit thereof, but you would do so without the wisdom you would otherwise acquire during the journey to find your true name."Oromis - Brisingr pgs. 641 and 642
Elves instinctively know their true name, and the human Dragon Riders usually went on quests to discover it or find an Elf to tell them, although this was rare.
When Eragon is captured by Durza, he makes up a false "true name" to fool him: Du Sundavar Freohr. This means "Death of the Shadows" in the Ancient Language. It is unlikely that this is in fact his true name. Galbotorix knows Murtagh's and Thorn's true names, and because of that he is able to control them at his free will. In Brisingr, Eragon derives Sloan's true name and uses it to bind him to an oath to stay away from Katrina and Roran and to go on a journey to Du Weldenvarden where he can live until he truely feels sorrow and remorse for what he did. This true name of Sloan's is made up from three words from the Ancient Language, Gramarye, which describe Sloan as a person.
In Brisingr, Eragon asks Oromis if he would be able to tell him his true name. While Oromis says he would probably be able to figure it out he tells Eragon that finding out your true name only has meaning if you do it by yourself. It has been speculated that Eragon will learn his true name in Book IV and speak it at the Rock of Kuthian to open the Vault of Souls. It has also been hypothesized that Eragon's true name is Brisingr, however this has been refuted by many as most peoples true name's are several words long, as well as the fact that many times magicians have said Brisingr around Eragon, namely Brom, Rhunon and others - and yet he did not get any feeling of a weird sensation; "...walking over my grave." Said Sloan, Brisingr.
If a person changes who they are completely, it is possible for their true name to change as well. It has been theorised that in Book IV Murtagh and Thorn will be able to change their true names so that Galbatorix's hold over them is broken and they will be able to fight with Eragon against the Empire.
