Growing up at Llanddewi Rectory in Gwent, a few miles north of Caerleon-on-Usk, Machen found the unknown all about him. Caerleon, with its romantic associations of Celtic, Roman and Arthurian splendours lay cradled in an enchanted land. The radiant sight of the line of hills stretching from Mynydd Maen to Twyn Barlwm, encountered one winter morning in 1880, led the seventeen-year-old Machen to poetic composition. ‘I haven’t a notion why such a sight as that should send a man to pen and ink and paper,’ he explained at a banquet in his honour in Gwent in 1937; ‘but so it was, and with pen and ink and paper I have stayed ever since.’

Eleusinia (1881), the long ‘classical’ poem inspired by the beauties of Gwent, was later denounced by its author as ‘a horrible production’, but Machen realised its significance. ‘I chose the mysteries first and I chose them last,’ he wrote in old age.

His first major venture into the macabre stemmed from his boyhood memories of a lonely white house, Bertholly, gleaning through the trees of Wentwood, the ancient forest which rises north-east of Caerleon.

And for some reason, or for no reason, this house which stood on the boundaries and green walls of my young world became an object of mysterious attraction to me. It became one of the many symbols of the world of wonder that were offered to me, it became, as it were, a great word in the secret language by which the mysteries were communicated. I thought of it always with something of awe, even of dread; its appearance was significant of . . . I knew not what.

In 1890, as a young literary man in London with little reputation, though he had written The Anatomy of Tobacco (1884) and The Chronicle of Clemendy (1888) and had a number of translations to his credit, Machen’s childhood fascination with the lonely house fused with the dream he had fashioned of Caerleon and its environs, and The Great God Pan, written ‘in the intervals between severe literary cramps’, was the result. Published by John Lane in 1894, ‘when yellow bookery was at its yellowest’, the tale was ridiculed by a host of reviewers. The Manchester Guardian notice—‘The book is, on the whole, the most acutely and intentionally disagreeable we have yet seen in English’—has passed into legend. The story certainly reveals signs of a tyro hand. ‘Melodrama is undoubtedly present, and coincidence is stretched to a length which appears absurd upon analysis,’ commented one of its later admirers, H.P. Lovecraft, but there is much to praise. Helen Vaughan hardly appears in the narrative, but her shadow broods over it like an angel from the pit: no mean achievement.

Machen’s principal technique in the novella, which he would adopt for nearly all his weird tales, was to avoid explicit descriptions of visceral horrors and allow sinister hints to work on the reader’s imagination. Even Helen’s dissolution at the climax is intentionally cloudy and vague, and all her sins are shown as through a glass darkly. This reticence has probably alienated as many readers as it has attracted, but Machen seems to have realised that a mystery revealed no longer remains a mystery. In his novel Peter Whiffle: His Life and Works (1921) Carl Van Vechten applauded Machen’s restraint. ‘He keeps the thaumaturgic secrets as the alchemists were bidden to do,’ says Van Vechten’s dilettante protagonist. ‘Instead of raising the veil, he drops it. Instead of revealing, he conceals. The reader may imagine as much as he likes, or as much as he can . . .’

Machen went on to refine his technique, crafting more tales in the same vein. The Three Impostors (1895), the picaresque romance in which London becomes ‘Baghdad-on-the-Thames’, appeared when the nation had vociferously turned against decadent art and its practitioners in the wake of the scandal surrounding Oscar Wilde. Machen purloined the structure of Stevenson’s New Arabian Nights (1882) and its sequel The Dynamiter (1885), written in collaboration with Fanny Osbourne. Although Stevenson’s romances have their macabre side, they are soufflés compared with the diablerie and corruption of Machen’s pastiche. A century on, the two classic horror fantasies extracted from the work, the ‘Novel of the Black Seal’ and the ‘Novel of the White Powder’, are still winning and disturbing readers. Both illustrate Machen’s imaginative genius in adapting a fragment of myth or tradition and weaving a bewitching tapestry around it.