Jace and Clary rush to make a final stand, but when a traitor is revealed, Jace and Clary may have a bigger fight on their hands than expected. Meanwhile, Alec and Izzy must fight off a large mass of demons that have unexpectedly shown up, as Luke and Simon head to the Seelie Court to deal with some unfinished business with the Seelie Queen.
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- "Ember" - Katherine McNamara
- The episode was shot together with the nineteenth episode, though the episodes were still released separately. Filming ended on May 9, 2017.
- Jace's "I love you" to Clary in the scene where he was dying was an ad lib by Dom that was kept in.