Magic of Sins by Karolyn Ciseau

Her sin is his addiction.

In Virtue, there are two unbreakable rules. First: Never trust a Sin Mage. Second: Never stray from the path of virtue. Kaya has always followed these rules without question—until the day she receives a summons to the royal palace.
The princess seeks Kaya’s help because the Sin Mage, Caden Nox, has specifically requested her. Known as the King of the Underworld in Virtue, Caden is willing to betray his fellow mages—but only under one condition: Kaya must commit one of the Seven Deadly Sins for him.
Yet Caden Nox doesn’t just tempt others; he is temptation itself. Kaya soon finds herself caught in a dark game of seduction and deception, where the stakes are higher than she could have ever imagined—and winning is not an option.