Eragon

He leaned forward, eyes gleaming. “Until then, I will say this to discourage any experiments: magic takes just as much energy as if you used your arms and back. That is why you felt tired after destroying the Urgals. And that is why I was angry. It was a dreadful risk on your part. If the magic had used more energy than was in your body, it would have killed you. You should use magic only for tasks that can’t be accomplished the mundane way.”

Admonishments, Eragon, page 208

“I did.” Relaxing into a chair, Nasuada let her gaze slowly drift up and down Trianna. The sorceress lifted her chin under Nasuada’s examination. “I need to know: What is the most important rule of magic?”

Trianna frowned. “That whatever you do with magic requires the same amount of energy as it would to do otherwise.”

“And what youcan do is only limited by your ingenuity and by your knowledge of the ancient language?”

“Other strictures apply, but in general, yes. Lady, why do you ask? These are basic principles of magic that, while not commonly bandied about, I am sure you are familiar with.”

> > Pg 493, Eldest, hanging by a thread

Hurrying to his master’s side, Eragon supported the elf’s slight weight as Oromis hobbled to the brook that rushed headlong toward the edge of the Crags of Tel’naeír. “Now that you have reached this stage in your education, I can teach you one of the greatest secrets of magic, a secret that even Galbatorix may not know. It is your best hope of matching his power.” The elf’s gaze sharpened. “What is the cost of magic, Eragon?”

“Energy. A spell costs the same amount of energy as it would to complete the task through mundane means.”

Oromis nodded. “And where does the energy come from?”

“The spellcaster’s body.”

Red blade, white blade, Eldest.