Hekla's Children by James Brogden

Hekla's Children by James Brogden is difficult to pigeonhole. Is it a fantasy, thriller, horror story or all of these genres together?
It starts with a terrifying story from some prehistoric times when the evil spirit steals a child from the village. This evil spirit is called afaugh. The strongest and bravest warrior is sacrificed to combat the evil in the underworld and protect the human world.
Then the story jumps to modern times, a decade before the main narrative takes place, when one of the main protagonists, teacher Nathan Brookes, takes a group of four teenagers on a walk in the nature park. Nathan is preoccupied with his personal problems, as the woman he loves has just had an abortion and doesn't want to do anything with him. Out of selfishness, Nathan follows his ex, having neglected his students. The group of teens vanishes without a trace. Only a rucksack is found.
Then a day later, one of the girls reappears, starved, dehydrated and terrified. She claims to have no memory of what has happened to her and the other three.
Ten years later, Nathan is trying to forget his past, doing another outdoorsy job, when the news of the body found in the ancient woodlands is broadcasted. Could it be one of the missing children?
Tara Doumani, a lecturer in osteoarchaeology, is requested to help with the police investigation. The carbon dating tests come with the proof that the body preserved in the peat is thousands years old.

The beginning of the book reads like a thriller or crime fiction, but soon it starts descending into a fantasy-horror story, with very dark undertones. The surreal blurs with the real world. Good and evil become intertwined, with characters showing both sides.
You get lost in the bleak landscapes of Un, "the place that is no place", some sort of the spirit realm.

The book raises a lot of moral questions, and challenges our perceptions of the world: what is real, how are we connected to the past, can we escape the past or change it?
The plot is compelling and clever, with twists getting darker and more menacing, and the story is imaginative.
I picked the book in The Works, thinking it would be something like Scandi Noir, totally not expecting where it would go or that it would move into the fantasy genre.

Without giving any spoilers, it was hard to understand why the protagonist turned out the way they did.
So, it was not what I expected at all.
I'm not going into the whole "Was it better?"-debate, and will answer like Ross to Rachel, it was different.