The Red Scrolls of Magic

The book is set during the events of City of Fallen Angels. After the Mortal War, High Warlock Magnus Bane and Shadowhunter Alec Lightwood decide to take a romantic vacation across Europe. 

Their trip is interrupted when Magnus learns about a demon-worshipping cult called The Crimson Hand. To his horror, the cult claims to have been founded by Magnus himself as a joke years ago. Now, it has grown into a dangerous organization intending to cause global chaos. Magnus and Alec must track down the cult’s leaders to stop them while keeping their mission a secret from the Clave.