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

Now we see which way the decision tips , she thought.
Slowly, Mapes reached into the neck of her dress, brought out a dark sheath. A black handle with deep finger ridges protruded from it. She took sheath in one hand and handle in the other, withdrew a milk-white blade, held it up. The blade seemed to shine and glitter with a light of its own. It was double-edged like a kindjal and the blade was perhaps twenty centimeters long.
"Do you know this, my Lady?" Mapes asked.
It could only be one thing, Jessica knew, the fabled crysknife of Arrakis, the blade that had never been taken off the planet, and was known only by rumor and wild gossip.
"It's a crysknife," she said.
"Say it not lightly," Mapes said. "Do you know its meaning?"
And Jessica thought: There was an edge to that question. Here's the reason this Fremen has taken service with me, to ask that one question. My answer could precipitate violence or . . . what? She seeks an answer from me: the meaning of a knife. She's called the Shadow in the Chakobsa tongue. Knife, that's "Death Maker" in Chakobsa. She's getting restive. I must answer now. Delay is as dangerous as the wrong answer .
Jessica said: "It's a maker—"
"Eighe-e-e-e-e-e!" Mapes wailed. It was a sound of both grief and elation. She trembled so hard the knife blade sent glittering shards of reflection shooting around the room.
Jessica waited, poised. She had intended to say the knife was a maker of death and then add the ancient word, but every sense warned her now, all the deep training of alertness that exposed meaning in the most casual muscle twitch.
The key word was . . . maker .
Maker? Maker .
Still, Mapes held the knife as though ready to use it.
Jessica said: "Did you think that I, knowing the mysteries of the Great Mother, would not know the Maker?"
Mapes lowered the knife. "My Lady, when one has lived with prophecy for so long, the moment of revelation is a shock."
Jessica thought about the prophecy—the Shari -a and all the panoplia propheticus, a Bene Gesserit of the Missionaria Protectiva dropped here long centuries ago—long dead, no doubt, but her purpose accomplished: the protective legends implanted in these people against the day of a Bene Gesserit's need.
Well, that day had come.
Mapes returned knife to sheath, said: "This is an unfixed blade, my Lady. Keep it near you. More than a week away from flesh and it begins to disintegrate. It's yours, a tooth of shai-hulud, for as long as you live."
Jessica reached out her right hand, risked a gamble: "Mapes, you've sheathed that blade unblooded."
With a gasp, Mapes dropped the sheathed knife into Jessica's hand, tore open the brown bodice, wailing: "Take the water of my life!"
Jessica withdrew the blade from its sheath. How it glittered! She directed the point toward Mapes, saw a fear greater than death-panic come over the woman. Poison in the point? Jessica wondered. She tipped up the point, drew a delicate scratch with the blade's edge above Mapes' left breast. There was a thick welling of blood that stopped almost immediately. Ultrafast coagulation , Jessica thought. A moisture-conserving mutation?
She sheathed the blade, said: "Button your dress, Mapes."
Mapes obeyed, trembling. The eyes without whites stared at Jessica. "You are ours," she muttered. "You are the One."
There came another sound of unloading in the entry. Swiftly, Mapes grabbed the sheathed knife, concealed it in Jessica's bodice. "Who sees that knife must be cleansed or slain!" she snarled. "You know that, my Lady!"
I know it now , Jessica thought.
The cargo handlers left without intruding on the Great Hall.
Mapes composed herself, said: "The uncleansed who have seen a crysknife may not leave Arrakis alive. Never forget that, my Lady. You've been entrusted with a crysknife." She took a deep breath. "Now the thing must take its course. It cannot be hurried." She glanced at the stacked boxes and piled goods around them. "And there's work aplenty to while the time for us here."
Jessica hesitated. "The thing must take its course ." That was a specific catchphrase from the Missionaria Protectiva's stock of incantations—The coming of the Reverend Mother to free you .