“I know about parabatai,” said Magnus, an angry, dark undercurrent to his voice. “I’ve known parabatai so close they were almost the same person; do you know what happens, when one of them dies, to the one that’s left —?”

“Alec isn’t happy,” said Magnus, as if she hadn’t spoken.“Of course he isn’t,” Isabelle snapped. “Jace—”“Jace,” said Magnus, and his hands made fists at his sides. Isabelle stared at him. She had always thought that he didn’t mind Jace; liked him, even, once the question of Alec’s affections had been settled. Out loud, she said:“I thought you were friends.”“It’s not that,” said Magnus. “There are some people — people the universe seems to have singled out for special destinies. Special favors and special torments. God knows we’re all drawn toward what’s beautiful and broken; I have been, but some people cannot be fixed. Or if they can be, it’s only by love and sacrifice so great it destroys the giver.”Isabelle shook her head slowly. “You’ve lost me. Jace is our brother, but for Alec — he’s Jace’s parabatai too —”“I know about parabatai,” said Magnus, his voice rising in pitch. “I’ve known parabatai so close they were almost the same person; do you know what happens, when one of them dies, to the one that’s left—”“Stop it!” Isabelle clapped her hands over her ears, then lowered them slowly. “How dare you, Magnus Bane,” she said.“How dare you make this worse than it is —”“Isabelle.” Magnus’ hands loosened; he looked a little wide-eyed, as if his outburst had startled even him. “I am sorry. I forget, sometimes . . . that with all your self-control and strength, you possess the same vulnerability that Alec does.”“There is nothing weak about Alec,” said Isabelle.“No,” said Magnus. “To love as you choose, that takes strength. The thing is, I wanted you here for him. There are things I can’t do for him, can’t give him . . .” For a moment Magnus looked oddly vulnerable. “You have known Jace as long as he has. You can give him understanding I can’t. And he loves you.”“Of course he loves me. I’m his sister.”“Blood isn’t love,” said Magnus, and his voice was bitter. “Just ask Clary.”

“Will closed his eyes. He could not hear Jem go, not anymore; he did not want to know the moment when he left and Will was alone, did now want to know when his first day as a Shawdowhunter without a parabatai truly began. And if the place over his heart, where his parabatai rune had been, flared up with a sudden burning pain as the door closed behind Jem, Will told himself it was only a stray ember from the fire.”

“Jem touched the parabatai rune on his shoulder, through the thin material of his nightshirt. "I am not alone," he said. "Wherever we are, we are as one.”

“It’s me,” he said, and cleared his throat. “I could understand if you didn’t believe me, but I swear on the Angel, Iz, it’s me.” Alec said nothing, but his grip on Jace’s hand tightened. “You don’t need to swear,” he said, and with his free hand touched the parabatai rune near his collarbone. “I know. I can feel it. I don’t feel like I’m missing a part of me anymore.”“I felt it too.” Jace took a ragged breath. “Something missing. I felt it, even with Sebastian, but I didn’t know what it was I was missing. But it was you. My parabatai.”

“The right hand doesn’t know what the left is doing” is a phrase that refers to times when people ought to know, but don’t know, about something that is happening very close to them. For instance, you ought to know about the man who watches you when you sleep.”