In December 2007, Matthias was among the many Shadowhunters forced into becoming a Dark Shadowhunter by drinking from the Infernal Cup. Inextricably bound and loyal to Sebastian Morgenstern, he obeyed his bidding blindly.
During the Great Council meeting, Matthias, under Jonathan's instructions, was able to infiltrate the Gard and the Council Hall. Dressed as a hooded guard, Matthias took everyone by surprise and threatened the life of Consul Jia Penhallow. He gave them a message from Jonathan, telling the attending members that Jonathan was all-powerful and as long as they did not give into Jonathan's demands, none of them would be safe. But if they gave him Jace and Clary, disaster would be averted.
While Jace surrendered himself, he insisted that Clary must come, as well, particularly since Jonathan was silently coaching Matthias with his actions. However, the minute he took the knife away from Jia's neck, Alec Lightwood shot the knife with an arrow, away from him. Chaos broke as Jia ran off and Jace charged towards him. Jonathan, however, had opened a fiery chasm that leads to Hell for his escape. Before disappearing, Matthias told them that Jonathan had given them two days to decide.
Matthias was a tall Shadowhunter who looked to be in his forties and had brown hair and thickly muscled arms.
- City of Heavenly Fire (first and only appearance)