Foul Days is the first book in the dark fantasy duology The Witch's Compendium of Monsters, written by Genoveva Dimova and published by TOR. Drawing heavily from Slavic folklore, we have a dark twisted adventure, with an excellent atmosphere, following Kosara, a witch from the walled city of Chernograd, and her fight for survival against the Zmey, the Tsar of Monsters.

Persecuted by the Zmey, and after being betrayed by a friend, Kosara's only resource to flee is trading her shadow (the source of her powers) in exchange from illegal passage across the Wall to Belograd, a safe place for monsters. However, despite Belograd is a safe place, she starts experiencing a fast-acting version of the deadly illness that affects shadowless witches; her only cure is tracking down the smuggler that took her across the Wall.
When she finds the death of the smuggler, with all the hints pointing towards Chernograd, she will have to team up with the Belogradian detective (Asen) that is investigating the assassination; both will have to collaborate if they want to recover Kosara's shadow and survive.

I found Kosara to be a great character, sarcastic at points, but still somebody that wants to use his powers to protect Chernograd's people, her people; with time running down, she will have to swallow her pride and collaborate with Asen, especially as the Zmey seems to be the one holding her shadow. Despite being young, she has seen too much, and that is reflected in the ways she behaves; confidence needs to be gained.
In comparison, Asen may lack that knowledge, but he's guided by a strong sense of duty; even if that means breaking the rules. His relationship with Kosara doesn't start in the best way, but slowly, confidence gets gained as both are struggling with guilt, and share the objective of protecting the people of their respective cities. The slow burn relationship between both (I'm not sure I would call it romantic) is simply sweet.
The rest of the characters are less defined, outside of exceptions such as the own Zmey; it is interesting how the Zmey, apart from being the Tsar of Monsters, is practically the archetype of the toxic love, that partner that covers you in love to, subsequently, show its true face and become the most possessive and abusive individual.

The worldbuilding is simply excellent, drawing from that Slavic folklore, and including many of the creatures; most of them appear in encounters that have an episodic feeling like in The Witcher. I would like to point that the dichotomy between Chernograd/Belograd can also remember to East Berlin/West Berlin, separated by a wall that not only keeps the monsters inside, but the people; Chernograd rooted in tradition and with many inhabitants that drink or smoke to deal with their difficult situation.
The pacing is fast, without a single dull moment, but allowing some times to recover the breath between high-tension moments; and Dimova's writing enhances the reading experience, giving us many memorable passages.

Foul Days is an excellent novel, a great example of modern dark fantasy that uses folklore not only to build the world, but to also include a thoughtful social commentary. Dimova's debut is a candidate to be my book of the year, and I can't wait to read Monstrous Nights, the second book in this Witch's Compendium of Monsters.