A Crown of Thorns and Shadow

Why these tropes fit

An assassin disguised as a beacon of purity is dispatched as a bride to infiltrate and assassinate the enemy king, only to find him a self-sacrificing ruler rather than the beast of legend. Trapped in forced proximity within the castle's shadows, her hidden identity collides with a slow-burn, dark, and steamy enemies-to-lovers arc complicated by morally gray loyalties, political intrigue, and magic that exacts a physical toll. The heroine's divided mission to kill the king or save her captive sister forces a betrayal that could cost her life or the world.

Evidence from the source

• Enemies to Lovers: “A slow burn enemies to lovers romance that eventually ignites.” source
• Forced Proximity: “Forced proximity in a castle full of secrets and shadows.” source
• Slow Burn: “A slow burn enemies to lovers romance that eventually ignites.” source
• Arranged Marriage: “she is offered as a bride to the enemy King” source
• Morally Gray: “A morally gray hero who would burn the world to keep her safe.” source
• Dark Romance: “dark fantasy romance” source
• Strong Heroine: “She is the assassin sent to end him.” source
• High Heat: “This steamy fantasy romance features: A slow burn enemies to lovers romance that eventually ignites.” source

Metadata source: Google Books. Quotes are used only to verify the tags.