What is the Secret Fire of Iluvatar?

Melkor deems himself God and wants to create Being.

Before aught else was made, Iluvatar sent the Secret Fire to burn at the heart of the world, and the vision of the world came alive (Ea).

The Secret Fire gave Being to the vision of the Ainur, and Iluvatar set this Being amid the Void – as the light shining in the darkness.

Melkor is ever seeking after the Secret Fire (Flame Imperishable) but cannot find …