Taking energy was an extremely advanced mental power taught only to the wisest and strongest of the Dragon Riders. It was a more advanced form of energy sharing and a less advanced form of element assimilation.
When he wished to gain more energy, the user of the mental power (dragons could learn it as well as magicians) would enter the mind of whoever they wanted to take energy from, and drain it out. They could use the energy either to cast a spell immediately, or transfer it into their bodies (or another receptacle, such as a gem) for later use.
Taking energy was one of the most useful mental powers. The user would have access to an almost endless supply of power in the right situations. The flow of energy could also be reversed, allowing the user to transfer energy into a weakened comrade. When perfected, this art could greatly aid a magic wielder.
Unfortunately, there were some drawbacks to using it effectively. The flow of energy was relatively slow, and thus not an efficient way to kill an enemy. Because it was a mental power and relied on breaking into an enemy's mind, anyone trained to defend themselves against a mental attack could block the drain. There were also wards that, while not protecting people's minds, could prevent their energy from being drained from their bodies. It also took time to master, and until then, it could also be dangerous. A careless magician could accidentally draw energy from his allies as well as his enemies, or kill whoever is providing the energy if they take too much. While using this skill in Du Weldenvarden, Eragon killed many lifeforms in the forest after taking energy from them, and nearly drained the energy from Oromis as well.
One can also absorb energy from the mind of a person or animal who had recently died, as demonstrated by Eragon when he took energy from a soldier that had recently died in the battle of Feinster in "Brisingr."
Role in the books Edit
Oromis taught Eragon how to draw energy from his surroundings after Eragon's mental powers grew strong enough. Afterwards, Eragon used it to refresh himself during the Battle of the Burning Plains and before and after the assault on Helgrind.
Glaedr also was capable of using this power. When Saphira arrived in Ellesméra, exhausted from the journey, the gold dragon transferred his energy into Saphira to refresh her. Later, when Oromis was struck down by Murtagh, Glaedr tried to save him by giving him energy, but failed.