As those who have read Inheritance should know, Murtagh helps decide the final confrontation versus Galbatorix through use of the Word, the ancient language's true name that so rings of metamathematics.
However, it doesn't seem to explain how did Murtagh know what the Word was in the first place. When Galbatorix first used it to undo Eragon's oath to kill him, the Word never stuck in Eragon's, or others', minds. After all, it would be plenty strange if Galbatorix would let anybody on the only way to undo every spell he's ever cast and every oath of fidelity hes ever received, so I doubt Galbatorix ever let anybody else on the Word willingly.
How does Murtagh know the Word then?