“Light, I need light!” gasped Aviel.

He was coming for her; he was desperate to stop her from reaching the summon stone, the source of his power.

She pounded half a dozen of the tarred boards until they split, bound the pieces into a bundle and set the end alight. The black wood blazed three feet high, making wraith shapes dance and whirl on the stone walls of the mill.

She had to get out, now. She jerked her boots on so violently that she wrenched her ankle and the pain brought tears to her eyes; today was going to be a very bad day. Aviel hobbled out to Thistle. He must have been woken by her scream, for his eyes were wide and fearful. He whinnied and tossed his head and kicked out.

“Good boy,” she said, stroking his muzzle. “Practise that kick; we’re going to need it.”

She led him to a block of stone and used it to mount, holding her blazing bundle high. Thistle kept moving; he didn’t like the flames. She directed him back to the track and turned his head downhill.

“Down,” she said.

The brand burned down to a stub. She tossed it into a stream and black night enveloped her. She laid a hand on Thistle’s warm neck.

“You’ll smell him a mile off. You’ll warn me, won’t you?”

The night was overcast and she could not see a single star. Aviel had no way of telling what time it was, though she still felt dull-headed; perhaps she had not slept long at all.

How far away was he? She thought he had been in a high place, looking down, though every ridge around here was a high place. He could have been five minutes away, or five hours.

The forest was full of small noises – the steady thump, thump of a bounding animal, a swooping flap that might have been an owl or even a bat, the rustle of small branches rubbing in the treetops. A tiny cry cut short, perhaps the owl taking a rat.

Those sounds had nothing to do with him. He would not come quietly through the black night; he would rampage through the forest, smashing down everything in his path.

She would have plenty of warning, though Aviel did not see how it would help her. Since she had to sleep, and evidently he did not, sooner or later he must catch her, and she had no way of defending herself against such a man. Her only hope was to outrun him, but first she had to know where to run to.

She had to use the Eureka Graveolence again.

Aviel did not want to. Her first experience had been most unpleasant, and the grimoire said the effects of the Great Potions were liable to be more dangerous each time they were used. But she had no choice.

She removed the cloth-wrapped phial from the last of her belt loops and checked it with her fingers. A thread was tied around it; it was the right potion. She took the stopper out, dampened the cloth with the potion and put the phial away. She could not risk losing it if something went wrong.

Very gingerly, she sniffed the cloth.

Thud-thud. It was as if Thistle had kicked her in the belly. The pain was so agonising that she doubled over, howling and gasping. The dark forest lit up, the tree trunks glowing a luminous white and bending down to trap her in a cage of live branches.

They toppled as if a thousand trees had been mown down by a single blow from a scythe. Lightning split the sky in two, the halves opened, and she saw that ugly ruined tower again, a long way off to her left. It had to be the place.

Minutes passed before Aviel was capable of sitting upright. Her belly was aching, and the muscle she had torn the previous night caused shards of pain with every movement. The darkness closed in, though she saw that the overcast was clearing away from the west, the stars coming out. She fixed on the direction where she had seen the tower and looked for a star she could use as a pointer.

If she took the angle between the red Scorpion Nebula and the yellow Triplets it would always point north, and the broken tower had been almost due north from here. It was enough to go on. When she came close the tower should be easy to spot – it was on the peak of a horn of bare rock.

She rode through the night, using the nebula and the Triplets to maintain her heading, and after a while Thistle got the message. Aviel dozed in the saddle, and whenever she roused he was heading in the right direction.

The sun rose. She emerged from forest onto a grassy plateau, a narrow strip of sloping land bounded on her left by rugged hills rising to high mountains, and on the right by a cliff hundreds of feet high. The plateau ran north, sometimes widening to five or six miles, sometimes pinching down to a rind only a few hundred yards across. It made it easy to navigate. As long as she kept to the plateau she would be heading roughly in the right direction.

It also made it easy for her hunter to follow.

In the mid-morning Aviel had to stop for a toilet break and a brief sleep. There was no sign of anyone behind her so she dismounted in the most open area she could find. She lay down in her sleeping pouch, closed her eyes and was drifting into a desperately needed sleep when she caught her hunter’s putrid odour: stronger, fouler, closer.

Get away while you can!

She scrabbled out of the sleeping pouch, blind with panic. Abandoning her pot and pan, her knife and spoon and even the sleeping pouch, she half ran, half hopped towards Thistle, hauled herself into the saddle and snatched at the reins.

“Go, Thistle!” she screamed. “Run as though the greatest demon of the underworld is after us.”

Thistle took her at her word and bolted north along the plateau, far faster than Aviel had ever gone before. She hunched down, hooked her cold fingers under the saddle and tried to stay on as he leaped rivulets and cascades, swerved around boulders that had rolled down from the rearing mountains on her left, and tore through patches of scented shrubbery.

Finally he slowed to a canter, trotting through a forest and past a still black lake with a half-ruined pavilion, then up to the foot of a steep, bare ridge that no horse could have climbed.

Thistle stopped. His great chest was heaving and his flanks were covered in overlapping trails of foam. He looked her in the eye as if to say, I dare go no further.

She stroked his muzzle. “Thank you, Thistle.”

Aviel dismounted stiffly, took her pack out of one saddlebag and put it on. It contained food and water, the hammer and flask of oil and the deadly grimoire. It was a cold day and would be a colder night. Why had she abandoned her sleeping pouch? Even if her enemy was riding, he must be hours behind.

She hugged Thistle around the neck. “You deserve a rest and the sweetest grass on the plateau. But keep a sharp lookout.”

She limped onto the ridge and began the climb. It was very steep and at first she had to go up on her hands and knees because she wasn’t able to stand up. But she knew where to go now, for she had fleetingly glimpsed the broken tower on the headlong ride.

She could not see it from here but knew she had a climb of many hours. If she could locate the stone, smash it to bits and burn it, she might stop the invasion. With any luck that would hurt her hunter too… No, what was she thinking? The only luck she ever had was bad.

Up she went, ever up. The path turned into a sinuous track along the top of the ridge. It was only a few feet wide and there was a deadly fall of hundreds of feet to either side into gorges choked with boulders.

The way steepened again. Steps had been cut into the top of the ridge here, though they were broken, icy and littered with frost-shattered rock. Aviel’s bad ankle was so swollen that she could not stand upright and she had gone back to hands and knees. She dragged herself up the abrasive rock with bleeding fingers, but she was not going fast enough.

She kept detecting him – ever stronger, ever fouler, ever closer. He was far stronger than her and utterly obsessed. She had to rest every hundred yards now but knew he did not stop at all.

Suddenly, after a climb of about eight hours, she smelled him again and almost fell off the ridge. How could he be here already? She had to be imagining it. But she looked down and there he was, huge, red-faced, driving up at a reckless speed. He was where she had been an hour ago, though at the pace he was going he would be here in twenty minutes. Could she reach the tower in that time? At her current rate it would take her another hour.

Aviel tried to climb faster, but her strength was fading and the heart had gone out of her. What was the point in enduring all this agony when she had no hope left?