Back in 2013 there was an NHK drama series 火怨・北の英雄 アテルイ伝 .
In that title, I can't understand the meaning of the compound kanji "火怨", and I couldn't find any examples or reference online that didn't merely point back to the drama title. Nor any dictionary entries.
Maybe it's regional dialect or dated usage, as this takes place in the Tohoku region in the 8th century.
To me, it looks like a new compound coined by the author. It's not uncommon at all for a novelist to make up a new kanji word (see this for another example).
It may be a rare term actually used somewhere in the past and known to experts. Still, an ordinary Japanese speaker don't know this word, and would not bother to look this up in a dictionary anyway. The supposed meaning ("flaming grudge") is self-explanatory.