Today’s second glitch:
No, you don’t have to actually write about this character (although you’re welcome to use the original writing prompt as a writing prompt if you want to).
If you’re using they as a gender-neutral singular pronoun, the correct reflexive form is themself, not themselves. (If you don’t like using themself as the reflexive form, don’t use they as a singular pronoun.)
Get rid of the em-dash after die. There’s no reason for it to be there. If it’s meant to indicate a dramatic pause or whatever, use an ellipsis instead.
Delete the comma after flames. (A compound predicate — that’s the entire verb part of the sentence — doesn’t require a comma except where meaning would be unclear otherwise, which is not the case here. Ignore the prepositional phrases in the predicate, if that helps you make sense of it: They’re enveloped and warded. See?)
We’re talking about a single person, right? One person cannot be the descendants, plural, of anyone. Change to descendant.
Write about a character who finds themself trapped within a burning building. The catch? Just as they think they’re about to die, they’re enveloped by the flames and warded from further damage. Double catch? This character has no idea, but they’re the descendant of a forgotten race.