Hello everyone, first post here, forgive me if I'm doing something wrong here.

I've been wondering for a while, what's the full story behind the dragonfires? I know that initially it required a heir of St. Alessia to rekindle them after the death of each emperor/empress, and later on, for some reason, it suddenly required a heir of Tiber Septim to do this. But what happened when there wasn't a true emperor? Were daedra free to roam Nirn and open portals wherever they pleased? And how does that fit in with the lore from ESO, where the dragonfires were dimmed by Mannimarco's treachery? And also, I seem to remember reading in a few in-game books that the Septim bloodline didn't exactly stay pure during the third era, at a moment even having a dark elf empress, completely unrelated to Tiber Septim. Is the line in "of the dragonfires", where Akatosh tells Alessia that we'll find a way to defend us from the daedra, supposed to hint that there are other ways of keeping the dragonfires lit? Lots of questions I know, I'm just hoping that a lore-expert can clear up this subject for me a bit. Any answers are appreciated.

Tiber Septim was not, iirc, a direct descendant of Alessia. But he was dragonborn. It might not be so much the direct bloodline descendants as it is being dragonborn. Akatosh can decide, afaik, who is dragonborn, so that kind of leaves it open.

I'm sure one of the lore people will have the exact citations.

Yeah, Tiber Septim was nordic, while Martin and Uriel were Imperial, so a direct heir doesn't appear to have been necessary.

Tiber Septim was many things according to MK. His appearance race is unknown other then he appeared human.