Much is misunderstood about true flame. Dragons do NOT breathe True Flame (although rumor has it the Black Dragon does).

The word “True” for all labels is a rough translation. It is roughly “source,” and “pure” and “archetype.”

While to the naked eye it often seems like True Flame destroys things instantaneously, it is like any flame in that it effects the surface…which in the case of True Flame is usually very, very, fast.
The idea is that True Flame does effect everything…sooner or later (although rumors swirl that the Black Dragon is immune).

Things of note:
True flame is non-conservation of energy destruction. Everything it destroys ceases to exist. It is not clear (to even the gods) if this include the “soul” (and thus is a True Death) or not, but all sentient beings destroyed by True Flame have never reincarnated.

“In the wild” the only spell that has true flame is prismatic spray. It is only 1% or less of the spectrum but is still devastating.

Two materials occurring naturally that “resist” True Flame are Lithon and Ptroni.
Lithon “coating” as an example resists True Flame for 2 seconds.

 
As a “rule of thumb” Lithon will burn off at a millimeter per combat round (10 seconds) and will be “soft” for a short period of time below that (about 10 mm).

Ptroni will last 10 times longer than Lithon.

Generally speaking, True Flame is modestly frowned upon as overly destructive, but in practice it has been “If you have it, use it as necessary.”