From "Manual of Muad'Dib" by the Princess Irulan

As long as Chani lives, Paul will not see his duty , Jessica thought. She has given him a son and that is enough .
A sudden longing to see her grandson, the child whose likeness carried so much of the grandfather's features—so like Leto, swept through her. Jessica placed her palms against her cheeks, began the ritual breathing that stilled emotion and clarified the mind, then bent forward from the waist in the devotional exercise that prepared the body for the mind's demands.
Paul's choice of this Cave of Birds as his command post could not be questioned, she knew. It was ideal. And to the north lay Wind Pass opening onto a protected village in a cliff-walled sink. It was a key village, home of artisans and technicians, maintenance center for an entire Harkonnen defensive sector.
A cough sounded outside the chamber hangings. Jessica straightened, took a deep breath, exhaled slowly. "Enter," she said.
Draperies were flung aside and Gurney Halleck bounded into the room. She had only time for a glimpse of his face with its odd grimace, then he was behind her, lifting her to her feet with one brawny arm beneath her chin.
"Gurney, you fool, what are you doing?" she demanded.
Then she felt the touch of the knife tip against her back. Chill awareness spread out from that knife tip. She knew in that instant that Gurney meant to kill her. Why? She could think of no reason, for he wasn't the kind to turn traitor. But she felt certain of his intention. Knowing it, her mind churned. Here was no man to be overcome easily. Here was a killer wary of the Voice, wary of every combat stratagem, wary of every trick of death and violence. Here was an instrument she herself had helped train with subtle hints and suggestions.
"You thought you had escaped, eh, witch?" Gurney snarled.
Before she could turn the question over in her mind or try to answer, the curtains parted and Paul entered.
"Here he is, Moth—" Paul broke off, taking in the tensions of the scene.
"You will stand where you are, m'Lord," Gurney said.
"What . . . " Paul shook his head.
Jessica started to speak, felt the arm tighten against her throat.
"You will speak only when I permit it, witch," Gurney said. "I want only one thing from you for your son to hear it, and I am prepared to send this knife into your heart by reflex at the first sign of a counter against me. Your voice will remain in a monotone. Certain muscles you will not tense or move. You will act with the most extreme caution to gain yourself a few more seconds of life. And I assure you, these are all you have."
Paul took a step forward. "Gurney, man, what is—"
"Stop right where you are!" Gurney snapped. "One more step and she's dead."
Paul's hand slipped to his knife hilt. He spoke in a deadly quiet: "You had best explain yourself, Gurney."
"I swore an oath to slay the betrayer of your father," Gurney said. "Do you think I can forget the man who rescued me from a Harkonnen slave pit, gave me freedom, life, and honor . . . gave me friendship, a thing I prized above all else? I have his betrayer under my knife. No one can stop me from—"
"You couldn't be more wrong, Gurney," Paul said.
And Jessica thought: So that's it! What irony!
"Wrong, am I?" Gurney demanded. "Let us hear it from the woman herself. And let her remember that I have bribed and spied and cheated to confirm this charge. I've even pushed semuta on a Harkonnen guard captain to get part of the story."
Jessica felt the arm at her throat ease slightly, but before she could speak, Paul said: "The betrayer was Yueh. I tell you this once, Gurney. The evidence is complete, cannot be controverted. It was Yueh. I do not care how you came by your suspicion—for it can be nothing else—but if you harm my mother . . . " Paul lifted his crysknife from its scabbard, held the blade in front of him. ". . . I'll have your blood."
"Yueh was a conditioned medic, fit for a royal house," Gurney snarled. "He could not turn traitor!"
"I know a way to remove that conditioning," Paul said.
"Evidence," Gurney insisted.
"The evidence is not here," Paul said. "It's in Tabr sietch, far to the south, but if—"
"This is a trick," Gurney snarled, and his arm tightened on Jessica's throat.
"No trick, Gurney," Paul said, and his voice carried such a note of terrible sadness that the sound tore at Jessica's heart.
"I saw the message captured from the Harkonnen agent," Gurney said. "The note pointed directly at—"
"I saw it, too," Paul said. "My father showed it to me the night he explained why it had to be a Harkonnen trick aimed at making him suspect the woman he loved."
"Ayah!" Gurney said. "You've not—"
"Be quiet," Paul said, and the monotone stillness of his words carried more command than Jessica had ever heard in another voice.