The Red Scrolls of Magic Book by Cassandra Clare and Wesley Chu

All Magnus Bane wanted was a vacation—a lavish trip across Europe with Alec Lightwood, the Shadowhunter who against all odds is finally his boyfriend. But as soon as the pair settles in Paris, an old friend arrives with news about a demon-worshipping cult called the Crimson Hand that is bent on causing chaos around the world. A cult that was apparently founded by Magnus himself. Years ago. As a joke.