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

Is this betrayal? Paul wondered. Surely not Gurney!
Around the room they fought—thrust and parry, feint and counterfeint. The air within their shield bubbles grew stale from the demands on it that the slow interchange along barrier edges could not replenish. With each new shield contact, the smell of ozone grew stronger.
Paul continued to back, but now he directed his retreat toward the exercise table. If I can turn him beside the table, I'll show him a trick , Paul thought. One more step, Gurney .
Halleck took the step.
Paul directed a parry downward, turned, saw Halleck's rapier catch against the table's edge. Paul flung himself aside, thrust high with rapier and came in across Halleck's neckline with the bodkin. He stopped the blade an inch from the jugular.
"Is this what you seek?" Paul whispered.
"Look down, lad," Gurney panted.
Paul obeyed, saw Halleck's kindjal thrust under the table's edge, the tip almost touching Paul's groin.
"We'd have joined each other in death," Halleck said. "But I'll admit you fought some better when pressed to it. You seemed to get the mood ." And he grinned wolfishly, the inkvine scar rippling along his jaw.
"The way you came at me," Paul said. "Would you really have drawn my blood?"
Halleck withdrew the kindjal, straightened. "If you'd fought one whit beneath your abilities. I'd have scratched you a good one, a scar you'd remember. I'll not have my favorite pupil fall to the first Harkonnen tramp who happens along."
Paul deactivated his shield, leaned on the table to catch his breath. "I deserved that, Gurney. But it would've angered my father if you'd hurt me. I'll not have you punished for my failing."
"As to that," Halleck said, "it was my failing, too. And you needn't worry about a training scar or two. You're lucky you have so few. As to your father—the Duke'd punish me only if I failed to make a first-class fighting man out of you. And I'd have been failing there if I hadn't explained the fallacy in this mood thing you've suddenly developed."
Paul straightened, slipped his bodkin back into its wrist sheath.
"It's not exactly play we do here," Halleck said.
Paul nodded. He felt a sense of wonder at the uncharacteristic seriousness in Halleck's manner, the sobering intensity. He looked at the beet-colored inkvine scar on the man's jaw, remembering the story of how it had been put there by Beast Rabban in a Harkonnen slave pit on Giedi Prime. And Paul felt a sudden shame that he had doubted Halleck even for an instant. It occurred to Paul, then, that the making of Halleck's scar had been accompanied by pain—a pain as intense, perhaps, as that inflicted by a Reverend Mother. He thrust this thought aside; it chilled their world.
"I guess I did hope for some play today," Paul said. "Things are so serious around here lately."
Halleck turned away to hide his emotions. Something burned in his eyes. There was pain in him—like a blister, all that was left of some lost yesterday that Time had pruned off him.
How soon this child must assume his manhood , Halleck thought. How soon he must read that form within his mind, that contract of brutal caution, to enter the necessary fact on the necessary line: "Please list your next of kin ."
Halleck spoke without turning: "I sensed the play in you, lad, and I'd like nothing better than to join in it. But this no longer can be play. Tomorrow we go to Arrakis. Arrakis is real. The Harkonnens are real."
Paul touched his forehead with his rapier blade held vertical.
Halleck turned, saw the salute and acknowledged it with a nod. He gestured to the practice dummy. "Now, we'll work on your timing. Let me see you catch that thing sinister. I'll control it from over here where I can have a full view of the action. And I warn you I'll be trying new counters today. There's a warning you'd not get from a real enemy."
Paul stretched up on his toes to relieve his muscles. He felt solemn with the sudden realization that his life had become filled with swift changes. He crossed to the dummy, slapped the switch on its chest with his rapier tip and felt the defensive field forcing his blade away.
"En garde!" Halleck called, and the dummy pressed the attack.
Paul activated his shield, parried and countered.
Halleck watched as he manipulated the controls. His mind seemed to be in two parts: one alert to the needs of the training fight, and the other wandering in fly-buzz.