I suspect that Aura invoked Himmel's death specifically to coerce Frieren into going HAM against her undead army. Figure, we know that demons basically only use language as a tool, right? Well, if invoking Himmel's death successfully coerces Frieren into going old-school on Aura's undead army...
If I had to guess, I'd wager that the fire witch tribe was never actually wiped out, and the fire witch tribe (or, at least one fire witch) is a sort of a "woman behind the curtain" to the church Hunters.
Odds are I'm 100% wrong tho so......