Shadowhunters and Downworlders are picking up the pieces after Valentine's attack at the Institute. Downworlder numbers are down and the different packs and clans debate on what they must do to survive. With Valentine in custody, the Institute is on high alert, but Jace carries the heaviest burden of all due to his part in igniting the Soul-Sword and the secret he is keeping from Clary. But Jace and the Shadowhunters don't have much time to mourn as a greater demon is loose in Manhattan. Meanwhile, Izzy struggles on the road to recovery when she encounters Sebastian, a mysterious stranger, as Simon tries to understand his new Daylighter status.
- Michael Reisz — writer
- Matt Hastings — director
- Filming began on January 23, 2017.
- The phrase "mea maxima culpa" in the title is Latin for "through my very great/most grievous fault."