There's a real gap in shelf-space between the sun-drenched romances and what I'm after. I keep finding urban fantasy where the supernatural politics are background noise for a relationship, not the main engine of dread. A book that actually delivers on that 'dark alliance' premise is 'Empire of the Vampire' by Jay Kristoff. It’s relentlessly grim, and the magic feels less like a tool and more like a curse everyone’s bargaining with. The vampire protagonist's uneasy partnerships, often with witches or heretic priests, are built on mutual hatred and necessity, not trust. You can feel the whole world sliding towards damnation, and every alliance just hastens it.
For a witch-centric take, 'The Year of the Witching' by Alexis Henderson comes close. It’s more about a young woman discovering her power in a puritanical, witch-hunting society, but the line between righteous and dark magic is constantly blurred. Her ‘alliances’ are with the spirits of the dark wood, and the cost is visceral. It’s less about two factions teaming up and more about an individual being consumed by the very forces she needs to survive. The atmosphere is so thick with gothic dread it almost makes up for the lack of a formal vampire-witch treaty.