The Succubus Bride (2001)

From the back of the book:

The war draws to a close. A year of bloody fighting and countless deaths has resolved nothing. The monster nation of Maldekore still occupies the mithral mines rightfully claimed by the Ralstein dwarves. Ulria, who has given refuge to the displaced dwarves, can no longer endure the burden of the war. Now the diplomats must settle what blood and steel could not.
The conflict can end with a simple ceremony a wedding between the ruling families of the warring nations. Ulria's prince accepts his fate until the demonic origins of the wizard Maldekore are revealed, casting doubts on the nature of his alluring sister. Can the adventurers retrieve a ring that will protect the young prince from his bethrothed, and preserve the required wedding, or will the peace shatter into senseless slaughter?

The Succubus Bride is an adventure for the d20 system that threatens the player characters with mortal danger and moral quandaries. For characters 7th-8th level.