Eragon: What did the Twins want me to do?
Arya: Something not even they can accomplish. It is possible to speak the name of an object in the ancient language and summon its true form It takes years of work and great discipline, but the reward is complete control over the object.— Eragon
The magician would say the True name of an individual or object and a copy of it/them would appear next to it/them. The magician would then have total command over their target.
One could command the being to do practically anything, or control an object utterly. One did not necessarily have to learn essence summoning, however, in order to give anyone a command using their True name. Essence summoning required years to master and Eragon was able to control Sloan without summoning his essence.
It is theorized that the being could kill themselves by willing themselves not to obey the command until their body fails. However, this seems highly unlikely, as Sloan, who vowed to resist Eragon's command to the death, obeyed him when he invoked his true name.
Role in the booksEdit
When testing Eragon, the frustrated Twins gave him a test they knew would be beyond him: summoning the essence of silver. Arya arrived just before Eragon tried to summon the essence and stopped the magicians, scolding them for trying to make Eragon do something he wasn't experienced enough to do, something even they couldn't do. She then demonstrated her own powers by summoning the essence of silver herself.
Much later, Eragon spoke of the subject of essence summoning with Glaedr, asking why Oromis had not covered the topic in his training. Glaedr explained that it was highly advanced magic but of little practical use and so there was no need for Eragon to try it at the time. Eragon then expressed a desire to use the spell, feeling that if there was one aspect of magic that he could do as well as anyone else could, then he might have a chance of defeating Galbatorix. With reluctance, Glaedr agreed, thus Eragon performed the spell under his observation and Saphira's. He successfully cast the essence of his Rider's sword, Brisingr and admired the perfection of the duplicate Sword before ending the spell.
Glaedr explained that one did not need to use the true name of the object to perform this spell, so long as they fully understood the nature of the object they were summoning. The name merely serves as a label for the knowledge. Eragon completed the spell simply by stating "Brisingr" and making his intentions clear in his mind.