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

What does she fear? Paul wondered.
The old woman studied Paul in one gestalten flicker: face oval like Jessica's, but strong bones . . . hair: the Duke's black-black but with browline of the maternal grandfather who cannot be named, and that thin, disdainful nose; shape of directly staring green eyes: like the old Duke, the paternal grandfather who is dead.
Now, there was a man who appreciated the power of bravura—even in death , the Reverend Mother thought.
"Teaching is one thing," she said, "the basic ingredient is another. We shall see." The old eyes darted a hard glance at Jessica. "Leave us. I enjoin you to practice the meditation of peace."
Jessica took her hand from Paul's shoulder. "Your Reverence, I—"
"Jessica, you know it must be done."
Paul looked up at his mother, puzzled.
Jessica straightened. "Yes . . . of course."
Paul looked back at the Reverend Mother. Politeness and his mother's obvious awe of this old woman argued caution. Yet he felt an angry apprehension at the fear he sensed radiating from his mother.
"Paul . . . " Jessica took a deep breath. ". . . this test you're about to receive . . . it's important to me."
"Test?" He looked up at her.
"Remember that you're a duke's son, "Jessica said. She whirled and strode from the room in a dry swishing of skirt. The door closed solidly behind her.
Paul faced the old woman, holding anger in check. "Does one dismiss the Lady Jessica as though she were a serving wench?"
A smile flicked the corners of the wrinkled old mouth. "The Lady Jessica was my serving wench, lad, for fourteen years at school." She nodded. "And a good one, too. Now, you come here!"
The command whipped out at him. Paul found himself obeying before he could think about it. Using the Voice on me , he thought. He stopped at her gesture, standing beside her knees.
"See this?" she asked. From the folds of her gown, she lifted a green metal cube about fifteen centimeters on a side. She turned it and Paul saw that one side was open—black and oddly frightening. No light penetrated that open blackness.
"Put your right hand in the box," she said.
Fear shot through Paul. He started to back away, but the old woman said: "Is this how you obey your mother?"
He looked up into bird-bright eyes.
Slowly, feeling the compulsions and unable to inhibit them, Paul put his hand into the box. He felt first a sense of cold as the blackness closed around his hand, then slick metal against his fingers and a prickling as though his hand were asleep.
A predatory look filled the old woman's features. She lifted her right hand away from the box and poised the hand close to the side of Paul's neck. He saw a glint of metal there and started to turn toward. . .
"Stop!" she snapped.
Using the Voice again! He swung his attention back to her face.
"I hold at your neck the gom jabbar," she said. "The gom jabbar, the high-handed enemy. It's a needle with a drop of poison on its tip. Ah-ah! Don't pull away or you'll feel that poison."
Paul tried to swallow in a dry throat. He could not take his attention from the seamed old face, the glistening eyes, the pale gums around silvery metal teeth that flashed as she spoke.
"A duke's son must know about poisons," she said. "It's the way of our times, eh? Musky, to be poisoned in your drink. Aumas, to be poisoned in your food. The quick ones and the slow ones and the ones in between. Here's a new one for you: the gom jabbar. It kills only animals."
Pride overcame Paul's fear. "You dare suggest a duke's son is an animal?" he demanded.
"Let us say I suggest you may be human," she said. "Steady! I warn you not to try jerking away. I am old, but my hand can drive this needle into your neck before you escape me."
"Who are you?" he whispered. "How did you trick my mother into leaving me alone with you? Are you from the Harkonnens?"
"The Harkonnens? Bless us, no! Now, be silent." A dry finger touched his neck and he stilled the involuntary urge to leap away.
"Good," she said. "You pass the first test. Now, here's the way of the rest of it: If you withdraw your hand from the box you die. This is the only rule. Keep your hand in the box and live. Withdraw it and die."
Paul took a deep breath to still his trembling. "If I call out there'll be servants on you in seconds and you'll die."
"Servants will not pass your mother who stands guard outside that door. Depend on it. Your mother survived this test. Now it's your turn. Be honored. We seldom administer this to men-children."
Curiosity reduced Paul's fear to a manageable level. He heard truth in the old woman's voice, no denying it. If his mother stood guard out there . . . if this were truly a test . . . And whatever it was, he knew himself caught in it, trapped by that hand at his neck: the gom jabbar. He recalled the response from the Litany against Fear as his mother had taught him out of the Bene Gesserit rite.
"I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain ."
He felt calmness return, said: "Get on with it, old woman."
"Old woman!" she snapped. "You've courage, and that can't be denied. Well, we shall see, sirra." She bent close, lowered her voice almost to a whisper. "You will feel pain in this hand within the box. Pain. But! Withdraw the hand and I'll touch your neck with my gom jabbar—the death so swift it's like the fall of the headsman's axe. Withdraw your hand and the gom jabbar takes you. Understand?"
"What's in the box?"
"Pain."
He felt increased tingling in his hand, pressed his lips tightly together. How could this be a test? he wondered. The tingling became an itch.
The old woman said; "You've heard of animals chewing off a leg to escape a trap? There's an animal kind of trick. A human would remain in the trap, endure the pain, feigning death that he might kill the trapper and remove a threat to his kind."
The itch became the faintest burning. "Why are you doing this?" he demanded.
"To determine if you're human. Be silent."
Paul clenched his left hand into a fist as the burning sensation increased in the other hand. It mounted slowly: heat upon heat upon heat . . . upon heat. He felt the fingernails of his free hand biting the palm. He tried to flex the fingers of the burning hand, but couldn't move them.
"It burns," he whispered.
"Silence!"
Pain throbbed up his arm. Sweat stood out on his forehead. Every fiber cried out to withdraw the hand from that burning pit . . . but . . . the gom jabbar. Without turning his head, he tried to move his eyes to see that terrible needle poised beside his neck. He sensed that he was breathing in gasps, tried to slow his breaths and couldn't.
Pain!
His world emptied of everything except that hand immersed in agony, the ancient face inches away staring at him.
His lips were so dry he had difficulty separating them.