The Darke Fog was rolling on. It had reached the door to Larry's Dead Languages. It seeped in around the edges, finding out the cracks, pouring through the knotholes, needling through the woodworm burrows. It gathered around the piles of translated papers, swirled into the much-repaired vase and snuffed out the candles in the window display that had been lovingly created by Beetle. It rolled on through the shop, up into the gallery, along the landing and up the rickety winding stairs. In his little room deep in the back of his house, Larry awoke. He sat up in bed and pulled the bedclothes around his chin. He stared into the darkness, listening hard. Something was wrong. Larry swung his sticklike legs out of bed and, as his bare feet flinched at the cold touch of the floorboards, he saw black smoke pouring underneath the door. Aghast, he leaped up - the house was on fire!

The smoke advanced toward him; it began to curl around his frozen toes and slowly, as if in a dream, Larry sat down again. A great feeling of contentment overwhelmed him. He was back in his old school, getting the Latin prize for the seventh time, and he had just seen his father in the audience, in the front row, smiling at him. Smiling at him. Larry. Clever Larry . . .

As the Darke Fog layered around him, Larry sank back onto the bed. His breathing slowed and, like a tortoise in the depths of winter, he slipped into a dark and dreamless state somewhere between life and death.

Marcia ushered Jillie Djinn and Marcellus, who had custody of Merrin, out into Wizard Way. She quickly locked the Manuscriptorium door behind her. Marcia could hardly bear to think about what she had left behind, but what was facing her was even worse. Advancing up Wizard Way like a pulsating black toad was a Darke shifting blackness.

Marcia was horrified to see that the rolling Fog was accompanied by a line of Things - the outriders of the Darke Domaine. Like the sweep of a terrifying search party, they spread out across Wizard Way, with the Fog tumbling behind. She stared in shock, unable to tear herself away from the disaster unfolding before her.

Marcellus tried to draw Marcia away. "Marcia, you must get to the Wizard Tower at once," he said.

Merrin's eyes flashed angrily at Marcellus. With the Darke Domaine advancing ever nearer he felt he was growing stronger. The Two-Faced Ring was growing hot on his thumb and the vicious green faces were beginning to glow. The top face winked up at Merrin, and suddenly he knew he could beat Marcia. He could beat them all. He was in charge now. He was the best.

First Merrin broke the Silent with the worst insult in the Castle, then he broke the Restrain. With a violent twist, he tore himself from Marcellus's grasp and delivered a vicious kick to the Alchemist's shins. As Marcellus hopped up and down, gasping in pain, Merrin raised his arms in the air and, in a taunting gesture, he pulled his wrists apart, snapping the Locking Band as if it were no more than tissue paper. Relishing his moment of triumph, Merrin darted forward and waved his left thumb in Marcia's face, laughing as she instinctively drew back. The ring's evil-looking faces glowered at her, their jade complexions gleaming.

Marcia knew that there was only one possible reason for Merrin's sudden surge of power - the oncoming Darke Domaine had indeed been Engendered by him. Up to that moment she had found it hard to believe that Merrin was capable of such a thing but now, as he pranced away, defiantly punching the air with his fist, with his Two-Faced Ring glittering, Marcia realized just how much control Merrin now had. It was a terrifying thought.

"You idiot!" she yelled at him. "You have no idea what you are messing with, do you?"

"Neither do you, Wizard-face." Merrin laughed. "Run away to your twinkly little Tower and take old haddock-brain with you. I don't need her anymore. See ya! Ha, ha, ha!" Merrin could hardly contain himself. He had never had such an attentive - such an astonished - audience. It was wonderful. It was what he had always wanted.

"That's what I think of your stupid Magyk!" he yelled at Marcia, flicking his fingers at her. Gesticulating and laughing, Merrin danced backward, his pale face lit by the still-burning torches and the ghostly candle displays shining onto the empty streets. "Come and get me if you dare!" he yelled.

Marcia did dare. It was undignified but she didn't care. Inside Merrin's nasty little stomach the precious half of the Paired Codes was churning, and she was not having her last chance to defeat him escape her. She tore down Wizard Way in pursuit. Merrin laughed and ran, his scribe's cloak streaming behind him, his outstretched arms flapping like a demented bird flying toward his flock.

Marcellus raced after Marcia. It was a long time since he had run anywhere and his shoes were not ideal for the job - particularly after their encounter with the Manuscriptorium door. But Marcia's pointy purple pythons were even less suited to running and he soon caught up with her.

"Marcia . . ." he puffed. "Stop."

Marcia shook Marcellus's hand off her arm. "Let go," she hissed.

Marcellus stood firm. "No. Marcia, don't you see? The closer you get to that" - he waved his free hand at the advancing Darke Domaine and its outriders - "the more power it gives him and the more it takes from you. Come away before something awful happens."

"Something awful has happened," snapped Marcia, setting off in pursuit once more.

Marcellus kept up with difficulty. "It could be worse . . . you still have the Wizard Tower . . . don't risk it all on a nasty little scribe."

Marcia stopped. "You don't understand - he's got the Paired Code!"

Marcellus looked shocked, but he quickly recovered himself. "You must leave the Code to its fate. You must go back to the Wizard Tower." His voice shook with urgency. "You must not lose that too."

"I shall lose neither." Marcia flared angrily. "Just watch me."

Marcellus and Marcia were now more than halfway down Wizard Way. Only a hundred yards or so in front of them, the wall of Darke Fog rolled slowly toward them. At the base of the Fog a line of Things stretched out, shifting and blending in with the Darke, loping slowly forward, pulling the Darke Domaine with them.

Merrin was heading erratically for the Fog. Spinning around to check that Marcia and Marcellus were still watching him, flashing rude signs, screaming obscenities, he drew ever closer to his Darke Domaine.

Marcia focused hard on Merrin, gauging the distance. Muttering the words for a Fast Freeze, she raised her arm and a streak of ice-blue light left her hand and arced into the air. It landed with a brilliant white flash in the middle of Merrin's back. He staggered forward and gave a loud cry.

"Good shot," muttered Marcellus.

Marcia grimaced. She had never before performed Magyk behind someone's back. It was considered the lowest form of Magyk, but now was not the time for such refinements. She had held back from Freezing Merrin, assuming she would get him to the Wizard Tower and deal with things there. Freezing someone was dangerous and not to be undertaken lightly. But now, with the lives of everyone in the Castle at stake, Merrin's safety was no longer a consideration.

Slowly Merrin turned around. Outlined in a blue-white crackle of the Freeze trying to take, he shivered and shook as though caught in an icy blast - but he did not Freeze. He stared at Marcia for some seconds, as though his brain had slowed and he was trying to work out what had happened. Marcia returned the stare, waiting impatiently for the Magyk to take effect. In the frost of the spell, Merrin shone out against the Darke Fog, but slowly he began to shine a little less. Horrified, Marcia saw the icy brilliance fade and Merrin shake himself, throwing off the Freeze like a dog throwing off water.

Marcia's Magyk had failed. It was then that she really understood what she was up against.

Marcellus stepped up beside her. "You must go now," he said quietly.

"Yes. I know," Marcia said, but she did not move.

Merrin was ecstatic - he had defeated the ExtraOrdinary Wizard. High on success, he turned to the line of Things and yelled, "Get her!"

Marcellus saw three Things step forward as one. He saw them take another step and that was all he waited to see. He grabbed Marcia's hand and ran, dragging her up Wizard Way, not daring to look behind. Breathless, they reached the Manuscriptorium, where Jillie Djinn was patiently, vacantly, waiting.

Marcia recovered her senses. She wheeled around to see how far away the Things were and saw to her great relief that they had barely moved. An encroaching Darke Domaine takes a lot of energy, and the Things were slow and ponderous. Knowing that it could do no more than cause a brief delay, Marcia threw an emergency Barrier across Wizard Way, then with the Chief Hermetic Scribe sleepwalking between them, she and Marcellus set off toward the Wizard Tower.

At the Great Arch an extremely anxious Hildegarde was hovering, waiting for Marcia's return.

"Madam Marcia! Oh, thank goodness you are here!"

Marcia wasted no time. "Is Septimus back?" she asked.

"No." Hildegarde sounded worried. "We thought he was with you."

"I feared as much." Marcia turned to Marcellus and laid her hand on his arm. "Marcellus. Please, will you find Septimus for me? And keep him safe?"

"Marcia, that is why I came to the Manuscriptorium. I am looking for him. I will not stop until I find him - I promise you."

Marcia gave Marcellus a strained smile. "Thank you. You know I trust you, don't you?"

"Well, I never thought I'd hear you say that," said Marcellus. "Things must be bad."

"They are," said Marcia. "Marcellus, if . . . if anything happens, I give you guardianship of my Apprentice. Farewell." With that she turned away abruptly and walked quickly into the dark blue shadows of the Great Arch, the tippy-tappy sound of her shoes echoing as she went.

Marcellus stood for a moment and watched something that he had only seen once before, in his first life as the Castle's greatest Alchemist. He saw the Barricade - a thick slab of ancient pitted metal - silently slice down through the center of the Great Arch, closing the main entrance into the Wizard Tower courtyard. It was, Marcellus knew, the first of many shields that would be sliding into place, readying the Tower for its strongest and most ancient Magyk of defense.

Next came the beginnings of a four-sided Living SafetyShield (this was the strongest SafetyShield possible; it was known as Living because it required the energy of many living presences within it to keep it active. It could also, in extremis, act independently). Like the Barricade, a Living SafetyShield was extremely rare. Marcellus watched it rise slowly from the walls surrounding the Wizard Tower courtyard, a blue shimmering skin that cast its eerie light into Wizard Way.

Satisfied that the Tower would be protected - for a while, at least - Marcellus slipped away, leaving Wizard Way to its fate. With his cloak blending into the shadows, the old Alchemist disappeared into the very narrowest of gaps between two ancient houses. Marcellus walked quickly through what, in his Time, had been known as the Canyons - formed in the earliest days of the Castle when the houses that lay between Wizard Way and the Moat were built. To protect against the spread of fire, houses had been built in blocks of two or three, with a tiny gap left between the blocks - a gap so small that Bertie Bott would not have been able to squeeze in. But Marcellus Pye moved fast through the Canyons like a snake down a pipe, heading for what he guessed was his last chance to find Septimus before the Darkenesse fell.