The Darkest Day - Britt Bury 2.5 Stars.

This could have easily been a 4 star book to me if the world-building had been better and if Izel had been more believable.

I found the set-up very difficult to follow. It's never made clear how the world has ended up with immortals and one remaining human. And why are the immortals fairly easy to kill? Stab them in the heart and decapitate them or kill their mate. That seems the opposite of what I consider immortal. But since I don't read a lot of PNR, maybe this is standard.
There were also times were it felt jarring to realize that cars and cellphones still existed. For much of the book, they are traipsing through dense forest on the way to an isolated castle. The hero Kelvin is armed with swords and knives. Why no guns? And the flimsy reason why they can't drive a car to the castle seems uninspired. Motorcycles, ATV's, helicopters are all reasonable alternatives since money doesn't seem to be an issue. Little details like this kept me from fully engaging with the story.
The romance was what kept me reading. Even though I am not a big fan of Fated Mates, there was enough spark and humor between Izel and Kelvin to make up for the confusing plot. At times, Izel bordered on Too Stupid To Live but when push came to shove, she showed herself to be capable. There were still some head-scratching moments as she metamorphasized from plain Jane Fionn with no emotions to killer sexpot bursting with insatiable lust.

I might try the next book in this series with the hope that some of the gaps of knowledge about this world are filled in or better explained because the author does do sexual and romantic tension fairly well.