The streets are dark now. I can see Caern crossing the square ahead of me. I don't want her to see me following, so I hang back. She's walking quickly, and it takes every ounce of my skill to keep her in sight and remain hidden from the Watch.

The people in the street give her a wide but polite berth. Keepers make common people like me nervous.

She runs down a back alley. Her pace has picked up now. I can see the faintest aura of blue around her. I chance a run across the well-lit road and down the alley after her.

I can see her halfway down. She's stopped, clutching at her head. The walls of the alley are tall and straight and a movement up there takes my eyes from her.

My heart beats faster. Directly above her head is a dark shape, perched on two window ledges across the alley. Time slows as I watch it unfurl eight long metal legs. It skitters down the wall straight for her.

It's a spider-construct, easily eight feet across. A fat black round body sprouts eight skeletal legs and two beady blue eyes. It snatches Caern off the floor with its front two legs. I hear her cry out weakly.

The spider-thing beats a tinny rhythm on the wall with its metal feet. It looks down at me. Its unmoving eyes seem triumphant. And it is gone, leaving curls of mist that quickly fade. I have a fire arrow in my hands that I don't remember drawing but it's too late to use it.

There's only one place she can be now. I turn in the direction of the Docks. Although my feet want to run I stay stealthy. I won't be much any good to her if the Watch get me.

I turn into the alley beside the sorcerer's house. Where the door was is now a featureless brick wall.

I remember the window above the altar. This is risky, too risky, but I don't want another death on my conscience. A water pipe runs up to the roof on the front of the building. It's wet and hard to grip but I start to climb anyway.

I haul myself over the ledge and crawl up the tiles. Two large closed shutters are ahead. I crouch next to them. The wooden slats are rotten and I pull two away so I can see in.

Caern is stretched on the altar. Her Keeper robe is gone and in its place is a trailing white robe. It too has taken on the blue aura. Her wrists and bare ankles are shackled in black metal, carved with gold sigils and runes. The sorcerer stands over her, reading from a book. I should have torched them when I had the chance. His constructs stand beside him, watching impassively.

She turns her head to the window, and though I know I am hidden I see recognition spark in her eyes. She concentrates. The glow waxes and wanes sharply. The shackles break, exploding into dust.

She floats upright. Her feet aren't touching the stone of the altar. The sorcerer squeals and falls back. His book falls with a dull thump.

She leaps toward the window and smashes the shutters. I duck. Splinters of wood rain down on my head. I almost avoid getting carried down the roof by her momentum but her hand closes on my shoulder. She pulls me with her.

We roll. At the bottom of the roof is a narrow ledge barely as wide as my shoulders. As the world spins I'm sickeningly sure we'll go over the edge but we stop as if we've hit a wall. All the air shoots out of me at the impact. I land on my back with her pinning me down. I struggle to breathe.

'It's happening,' she cries. 'You have to kill me now!'

I open my mouth to speak, but a roar from above cuts me off. The sorcerer appears at the window. He's enraged and I can see the fear in his eyes that he might lose his prize. Caern snaps her head around and glares at him. There's an animal cast to her eyes. She throws out a hand, palm facing toward him and fingers splayed. The sorcerer is engulfed in flames. He burns so brightly he leaves a white afterimage on my eye. When I blink it away, nothing is left of him but ash.

'Artemus has the Stone,' I say stupidly.

'It's too late,' she snaps. 'Even if I had it in my hands now it would be too late. Please, Garrett.' She stands, freeing me to stand too. I try not to look down. It's a long way to the street.

I don't want to do it.

She takes me dagger and presses it into my hands. 'Please!' She's crying, tears running freely down her cheeks. 'Before it's too late!' I can see the blue glow getting stronger around her until it forms a halo of light. A double image of her flickers, larger than she is. Caern stands at its centre, pleading.

I stand frozen, dagger in my numb fingers.

She can see I'm not going to do it. She puts a soft hand on my cheek and pushes herself into my mind. She lets me feel the full force of her emotion, her fear of dying and taking other people with her, her fear of being imprisoned. Now more than ever she doesn't want to let go of life. She tries to force my hand and it rises at her command, but I force it down. I won't let her take control.

It begins to rain heavily. Warm water runs down my face. My vision blurs and I blink hard.

'If you don't do it, you'll die too,' she says. She's still in my head and I hear the sly thought she's put into those words. She knows me too well now. Those should be reasons enough for me. I pull her close, hugging her tightly. For two heartbeats we stand there. I steel myself.

But it's over so quickly. I raise the dagger and stab her in the spine at the base of her neck. Blood spreads down her wet white dress. She goes limp in my arms. He knees buckle and she slips from my grasp, falling slowly down to the street. I am unable to tear my eyes away. As she lands the blue glow vanishes as if it had never been. She is just another body on the streets of the Docks.

I climb down the pipe again. Her body is sprawled across the wet cobbles. Her blood mingles with rainwater. Her gown is plastered to her skin.

Her head is tilted sideways, looking away from me. I can see the hilt of my dagger beneath her wet hair. I bend down, touch the leather binding, but take my hand back.

I leave it buried in her

I walk away.