Clockwork Princess

the puzzle—”
    “Is you,” said Mortmain.
    “You say that over and over, and yet you refuse to explain it,” Tessa said. “You demand my cooperation and yet you tell me nothing. You have me imprisoned here, sir, but you cannot force my speech with you, or my willingness if I choose not to give it—”
    “You are half-Shadowhunter, half-demon,” Mortmain said. “That is the first thing you should know.”
    Tessa, already half-turned away from him, froze. “That is not possible. The offspring of Shadowhunters and demons are stillborn.”
    “Yes, they are,” he said. “They are. The blood of a Shadowhunter, the runes on the body of a Shadowhunter, are death to a warlock child in the womb. But
your mother was not Marked
.”
    “My mother was not a Shadowhunter!” Tessa looked wildly to the portrait of Elizabeth Gray over the fireplace. “Or are you saying she lied to my father, lied to everyone all her life—”
    “She did not know,” said Mortmain. “The Shadowhunters did not know it. There was no one to tell her. My father built your clockwork angel, you know. It was meant to be a gift for my mother. It contains within it a bit of the spirit of an angel, a rare thing, something he had carried with him since the Crusades. The mechanism itself was meant to be tuned to her life, so that every time her life was threatened the angel would intervene to protect her. However, my father never had a chance to finish it. He was murdered first.” Mortmain began to pace. “My parents were not singled out for murder, of course. Starkweather and his kind delighted in slaughtering Downworlders—they grew rich off the spoils—and would take the slightest excuse to bring violence against them. For that he was hated by the Downworlder community. It was the faeries of the countryside who helped me escape when my parents were killed, and who hid me until the Shadowhunters stopped looking for me.” He took a shuddering breath. “Years later, when they decided to have their revenge, I helped them. Institutes are protected against the ingress of Downworlders, but not against mundanes, and not, of course, against automatons.”
    He smiled a terrible smile.
    “It was I, with the help of one of my father’s inventions, who crept into the York Institute and switched the baby in the crib there for one of mundane descent. Starkweather’s granddaughter, Adele.”
    “Adele,” Tessa whispered. “I saw a portrait of her.”
A very young girl with long, fair hair, dressed in an old-fashioned child’s dress, a great ribbon surmounting her small head. Her face was thin and pale and sickly, but her eyes were bright
.
    “She died when the first runes were put on her,” said Mortmain with relish. “Died screaming, as so many Downworlders had before at the hands of Shadowhunters. Now they had killed one they had come to love. A fitting retribution.”
    Tessa stared at him in horror. How could anyone think that to die in agony was fitting retribution for an innocent child? She thought of Jem again, his hands gentle on his violin.
    “Elizabeth, your mother, grew up not knowing she was a Shadowhunter. No runes were given to her. I followed her progress, of course, and when she married Richard Gray, I made sure I employed him. I believed that the lack of runes on your mother meant that she could conceive a child who was half-demon, half-Shadowhunter, and to test that theory I sent a demon to her in the shape of your father. She never knew the difference.”
    Only the emptiness in Tessa’s stomach kept her from being sick. “You—did
what
—to my mother? A demon? I am half-demon?”
    “He was a Greater Demon, if that comforts you. Most of them were angels once. He was fair enough in his own aspect.” Mortmain smirked. “Before your mother became pregnant, I had worked for years to finish my father’s clockwork angel. I finished it, and after you were conceived, tuned it to
your
life. My greatest invention.”
    “But why would my mother be willing to wear it?”
    “To save you,” said Mortmain. “Your mother realized that something was wrong when she became pregnant. To carry a warlock child is not like carrying a human child. I came to her then and gave her the clockwork angel. I told her that wearing it would save her child’s life. She believed me. I was not lying. You are immortal, girl, but you are not invulnerable. You can be killed. The angel is tuned to your life; it is
designed
to save you if you are