"Slaying the Vampire Conqueror" is a standalone and an installment of the Mortal Enemies to Monster Lovers series. I won't read the other books in the series, but this one caught my interest.

Sylina is part of the (cultish) Arachessen, who devote themselves to the Goddess of Fate, Acaeja, and 'sacrifice' their eyesight in order to see "the entire world". Despite not seeing, Sylina experiences the world through threads, which allows her to see more than the naked eye can. When a vampire conqueror, Atrius, arrives on Glaea's shores, Sylina is tasked with earning his trust, becoming his seer by infiltrating his army, and killing him. 

Right away, I was delighted to meet Sylina. She's a little lost and unsure about her place in the world, struggles internally, but is also absolutely badass - a trained assassin. Talk about duality! It was a complex journey for her: the past, her struggles and beliefs, the sisterhood and Sightmother, the war and her mission, and of course Atrius and her growing feelings.
A complicated mess that would cost everybody involved a lot, regardless of what Sylina decides to do.
Atrius was actually surprisingly refreshing; he wasn't your typical "villain", he had his reasons, and he was more... soft? Perhaps not the right word, but he was more human than some others. 

There was a lot of action, a few funny bits, a whole lot of tension, and too much heartbreak. Atrius and Sylina had a good connection; something was lacking, but I can't put my finger on it. I liked their "too many?" and "not for us" moments; they were such an entertaining and slaying fighter duo. Who needs an army when you've got these two?
I would've wanted to know more about their future: what happens to Nero, the Arachessen, how is everyone doing, and what about their work ahead? The ending could've been a little more, but it wasn't terrible.