"I am Theresa Gray, daughter of a Greater Demon and Elizabeth Gray, who was born Adele Starkweather, one of your kind. I was the wife of William Herondale, who was the head of the London Institute, and I was the mother of James Herondale and Lucie Blackthorn. Will and I raised our Shadowhunter children to protect mundanes, to live by the Laws of Clave and Covenant, and to keep to the Accords. Once, I lived among the Shadowhunters. Once I might almost have seemed like a person to you." –Tessa to Jocelyn in 1993, The Last Stand of the New York Institute
The Clockwork angel pendant was designed to protect Tessa. It was given to her mother by Mortmain. It contained the angel Ithuriel's soul.