[Child of the 7th Age]

Oh, dear, my head is spinning with all these fires and flames.

Even the folk who wrote the guide-books seem uncertain about a lot of this.

This is from Robert Forster:

"Udun" means the source of the dark flame of the Balrogs, "possibly Utomno", "possibly the darkness of the Spirit of Melkor. Udun in Sindarin means "un-West". It means the same as "Utumno" which is "not-valley" in Quenya. Utumno is the first dwelling of Melkor in Arda.

Getting confused? Here's more:

Flame Imperishable: the creating spirit of Iluvatar, by which the Ainur and Ea were made, possessed by Iluvatar alone. "Also called the Imperishable Flame, the Fire, and perhaps the Flame of Anor" (nice hedging there!) See also Secret Fire

Secret Fire: the Holy Spirit, the power giving substance and life to the Creation of Iluvatar. "It was this light, also called the Flame of Anor (perhaps loosely) that Gandalf served, and the evil followers of Melkor and Sauron envid and feared.) (Me--that perhaps loosely means they fdged again.)
Most probably the same as the Flame Imperishable.

Now my head hurts even more.

sharon, the 7th age hobbit.