What is the word for the name of names?

This is intentionally never revealed and likely doesn't even exist.

What is Eragon’s true name? What is the true name for the Ancient Language?

In regard to Eragon’s true name, I chose not to reveal it because those words are words of magic. I felt that giving them to readers would spoil some of the mystery and power they hold. You could say the whole Inheritance Cycle encompasses Eragon’s true name. But its short form is a secret between Eragon, Saphira, Glaedr, and Arya.

As for the name of the Ancient Language . . . that, too, is a secret.

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