A spider emerges from a crevice in the cold limestone wall and waits. Far below, amidst a tiny pool of flickering light, Father Thomas dips a quill into coal-black ink and stares absently at the squat candle. And then,biting his lip, he peers down at the word he has written. A memory puckers his brow and forces a grimace. He wipes a tear from his eye,inhales deeply through his nostrils and resumes the letter. The words splutter and falter, bunching into short desperate sentences. His tight lips twitch involuntarily, punctuating each syllable as if sewn together in macabre penance. When he is done, the priest folds the letter and with shaking hands slides it into an envelope. A rush of wind rattles the shutters and delivers an echo of cawing crows. Father Thomas kisses the envelope and makes the sign of the cross. The candle flame shrinks and snuffs to a glowing ember, unravelling an ethereal black snake that slithers up into the rafters, past the all-seeing spider and its silver web.

The bell for Lauds tolled across the courtyard, and Father Thomas realised he had not slept. He had sat hunched on the edge of his cot, the envelope pressed between his palms like a relic he was afraid to drop. When he did eventually rise he walked the long cloister with his eyes fixed on the flagstones, where the brothers passed him in silence, hoods drawn, and he could not tell whether their lowered gazes were reverence or knowledge. Perhaps it made no difference. In a place like this, a secret was only a rumour that hadn't yet found its mouth.

Brother Anselm was waiting at the gatehouse, a boy of nineteen with ink-stained fingers. He took the envelope with both hands.

"To the Bishop at Aldermoor," Father Thomas said. His voice cracked; he cleared his throat and tried again. "Into his hands only."

"Yes, Father." Anselm turned the envelope over and saw the wax seal, the crest of the order pressed deep into the crimson. His brow creased. "Is it grave, Father?"

"It is finished," said Thomas.

He watched the boy hurry through the gate and down the mud road until the mist swallowed him. Only then did he allow himself to lean against the cold stone and close his eyes.

Sister Anne.

He had tried all night not to think her name. It rose now regardless; he saw her as she had stood before the tribunal three weeks ago, small in her grey habit, hands folded, chin lifted. When they asked her to recant, she had looked at each of the six judges in turn, and then at him, with weepy eyes, and said quietly, "I cannot unsee what I have seen, Father. Would you have me lie to God to comfort men?"

The decree had been precise. He had written it in the language of necessity, each phrase a stone laid upon the last. Obstinate in error. Contumacious before the tribunal. A danger to the souls entrusted to her. And then the sentence: to be immured in perpetuity, walled within the chapel crypt, that she may repent in silence before the face of the Almighty.

That evening the chapter house filled with the dull murmur of men who knew something was coming. Father Thomas stood at the head of the long table, the six judges arranged along its sides like crows on a fence. Prior Benedict, silver-haired and serene, folded his hands over his stomach.

"It is done, then?" asked Benedict.

"The decree has gone to Aldermoor. The Bishop will seal it within the week."

A murmur passed among them.

"You have done a hard thing, Thomas," Benedict said. "A necessary thing. Two novices have already come to me, troubled by her story."

"Her abbey was burned," Thomas said. His mouth had gone dry. "She spoke only of what she claimed to have seen."

"And Abbess Margery says she is troubled beyond the Father's help." Benedict's tone remained gentle, which made it worse. "Mercy lies keeping the well healthy, not in saving the already dead."

Then why does it feel like murder?

He did not say it. He bowed his head instead and murmured the expected assent, and the others took it as confirmation. The brothers filed out, their lanterns shrinking down the corridor, until Thomas stood alone in the dark with the smell of tallow. Sleep would not come. At the hour when even the owls fell quiet, he found himself walking, drawn by something he refused to name, down the narrow spiral stair that led beneath the chapel.

The crypt was cold as a dead thing. He carried no lantern, only a single candle that bent and shivered in the draught, making the shadows of the stone lean int towards him. Along the far wall, a recess had already been marked in chalk, a rectangle just wide enough for a kneeling woman, and beside it, stacked neat as bread, the pale blocks that would close it. Thomas set the candle on the floor and pressed his palm flat against the stone. It was damp. Somewhere in the dark, water dripped with the patience of a clock.

"Forgive me," he whispered. He did not know whether he spoke to God, to the nun, or to the boy he had once been, the one who had taken his vows believing that faith was a lamp and not a blade.

The candle guttered. In the weak light, something caught his eye on the wall above the recess. He leaned closer. It was a mark, a small circle with rays reaching outward, cut by some forgotten hand.

A sun. Or an eye.

Behind him, softly, a footstep scraped on stone. Thomas turned. The stair was empty, a black mouth yawning upward.

"Who is there?"

Then, after a long silence, from very far above, the sound of a woman humming. He knew the tune. He had heard it only once, drifting from a cell window the night before the tribunal. It was a hymn he had never learned, in a language he could almost understand.