A mysterious principle or essence said to have been placed at the heart of the world, also known as the Flame Imperishable and (apparently) as the Secret Fire. Little is known for certain about the nature of the Flame, but it seems to have been a creative principle, allowing beings other than Ilúvatar himself to create things of their own.

From the earliest times, Melkor sought to control the Imperishable Flame and use it to his own ends. He never succeeded, and so he never acquired the power to truly create things of his own. For that reason, he was forced to fill his armies by corrupting beings who already in existed in Arda (so, for example, the Orcs were said to have originated from the earliest Elves).