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

Yueh! Leto thought. He's sabotaged the house generators! We 're wide open!
Yueh began walking toward him, pocketing a dartgun.
Leto found he could still speak, gasped: "Yueh! How?" Then the paralysis reached his legs and he slid to the floor with his back propped against the stone wall.
Yueh's face carried a look of sadness as he bent over, touched Leto's forehead. The Duke found he could feel the touch, but it was remote . . . dull.
"The drug on the dart is selective," Yueh said "You can speak, but I'd advise against it." He glanced down the hall, and again bent over Leto, pulled out the dart, tossed it aside. The sound of the dart clattering on the stones was faint and distant to the Duke's ears.
It can't be Yueh , Leto thought. He's conditioned .
"How?" Leto whispered.
"I'm sorry, my dear Duke, but there are things which will make greater demands than this." He touched the diamond tattoo on his forehead. "I find it very strange, myself—an override on my pyretic conscience—but I wish to kill a man. Yes, I actually wish it. I will stop at nothing to do it."
He looked down at the Duke. "Oh, not you, my dear Duke. The Baron Harkonnen. I wish to kill the Baron."
"Bar . . . on Har . . . "
"Be quiet, please, my poor Duke. You haven't much time. That peg tooth I put in your mouth after the tumble at Narcal—that tooth must be replaced, in a moment, I'll render you unconscious and replace that tooth." He opened his hand, stared at something in it. "An exact duplicate, its core shaped most exquisitely like a nerve. It'll escape the usual detectors, even a fast scanning. But if you bite down hard on it, the cover crushes. Then, when you expel your breath sharply, you fill the air around you with a poison gas—most deadly."
Leto stared up at Yueh, seeing madness in the man's eyes, the perspiration along brown and chin.
"You were dead anyway, my poor Duke," Yueh said. "But you will get close to the Baron before you die. He'll believe you're stupefied by drugs beyond any dying effort to attack him. And you will be drugged—and tied. But attack can take strange forms. And you will remember the tooth. The tooth , Duke Leto Atreides. You will remember the tooth."
The old doctor leaned closer and closer until his face and drooping mustache dominated Leto's narrowing vision.
"The tooth," Yueh muttered.
"Why?" Leto whispered.
Yueh lowered himself to one knee beside the Duke. "I made a shaitan's bargain with the Baron. And I must be certain he has fulfilled his half of it. When I see him, I'll know. When I look at the Baron, then I will know. But I'll never enter his presence without the price. You're the price, my poor Duke. And I'll know when I see him. My poor Wanna taught me many things, and one is to see certainty of truth when the stress is great. I cannot do it always, but when I see the Baron—then, I will know."
Leto tried to look down at the tooth in Yueh's hand. He felt this was happening in a nightmare—it could not be.
Yueh's purple lips turned up in a grimace. "I'll not get close enough to the Baron, or I'd do this myself. No. I'll be detained at a safe distance. But you . . . ah, now! You, my lovely weapon! He'll want you close to him—to gloat over you, to boast a little."
Leto found himself almost hypnotized by a muscle on the left side of Yueh's jaw. The muscle twisted when the man spoke.
Yueh leaned closer. "And you, my good Duke, my precious Duke, you must remember this tooth." He held it up between thumb and forefinger. "It will be all that remains to you."
Leto's mouth moved without sound, then: "Refuse."
"Ah-h, no! You mustn't refuse. Because, in return for this small service, I'm doing a thing for you. I will save your son and your woman. No other can do it. They can be removed to a place where no Harkonnen can reach them."
"How . . . save . . . them?" Leto whispered.
"By making it appear they're dead, by secreting them among people who draw knife at hearing the Harkonnen name, who hate the Harkonnens so much they'll burn a chair in which a Harkonnen has sat, salt the ground over which a Harkonnen has walked." He touched Leto's jaw. "Can you feel anything in your jaw?"
The Duke found that he could not answer. He sensed distant tugging, saw Yueh's hand come up with the ducal signet ring.
"For Paul," Yueh said. "You'll be unconscious presently. Good-by, my poor Duke. When next we meet we'll have no time for conversation."
Cool remoteness spread upward from Leto's jaw, across his cheeks. The shadowy, hall narrowed to a pinpoint with Yueh's purple lips centered in it.
"Remember the tooth!" Yueh hissed. "The tooth!"