Between April and May 1662, Isobel Gowdie, a Scottish housewife from Auldearn in Morayshire, shocked her small community by revealing she was a witch.

She made four confessions in all, recounting in fascinating and explicit detail most of the elements that make up our popular idea of what it means to be a witch. Gowdie spoke at length about the sabbath, the coven, her magical powers and, perhaps most interesting for contemporary readers, her licentious relations with the devil.

James MacMillan’s powerful work, The Confession of Isobel Gowdie, which is played by the London Symphony Orchestra tomorrow as part of the Proms season, has been called a kind of requiem for Gowdie, a memorial to the persecution that many women — and some men — caught up in the “witch craze” of the 15th, 16th and 17th centuries were subjected to. It is estimated that across Europe 40,000 died in these “burning times”. And when it came to torture and execution Scotland had few rivals.

Yet one of the strange things about Gowdie’s case, according to the account of her trial given in Robert Pitcairn’s Criminal Trials in Scotland (1833), is that her confessions were voluntary. She wasn’t persecuted; indeed, she carried on a secret life as a witch for 15 years without anyone noticing, not even her husband. She wasn’t subject to torture; the account states that she made her confessions “without persuasion”. Some historians believe she wasn’t executed.

None of this diminishes the brooding, moving beauty of MacMillan’s work. The composer’s interest in Gowdie attracted me for a number of reasons, not least because I’ve written several books about the occult and its influence on popular and “highbrow” culture.

But I have another thing in common with MacMillan. When I first became interested in magic and the occult, I was a musician, not classically trained, but a punk rocker living in New York City.

I was a founding member of Blondie, and in the band’s early days, circa 1975, I lived with Debbie Harry and her boyfriend, Chris Stein, who was the guitarist in the group. I played bass and wrote a few songs, one of which, (I’m Always Touched By Your) Presence, Dear, which is about telepathy between my girlfriend and me, became a big hit.

Harry had a casual interest in X-Files sort of things — she once told me she thought she was an alien. But Stein had a real penchant for black magic, voodoo, and satanic bric-a-brac, a kitschy obsession with collecting occult trinkets and books. One book caught my eye, and once I had started reading, I didn’t put it down for a week. It was The Occult by Colin Wilson.

In his chapter on witchcraft, which includes the case of the North Berwick witches (convicted of trying to murder James VI), Wilson discussed Gowdie. Speaking of the sexual character of much of her confession, and of her dull life on a farm, Wilson said that Gowdie seemed “a highly sexed girl being driven half insane with frustration, until she evolves a whole fantasy about the powers of evil”.

As a complete account of Gowdie’s confessions this is debatable, but there’s no argument that sex plays a powerful part in them. The devil, she tells us, “was a big, black, hairy man”.

“He would be with us like a stallion among mares,” claimed Gowdie. “The youngest and lustiest women would have very great pleasure with him, much more than with their own husbands . . . He was more satisfying for us than any man can be.”

This is echoed by Janet Breadheid, a woman Gowdie named as one of her coven and who also confessed voluntarily. If it was only sexual fantasy, who could blame her? Married to a plodding farmer, who for 15 years didn’t have a clue about her secret life, things must have been pretty dull for Gowdie.

On the nights she headed out, she left a broomstick beside her husband in bed: he never knew the difference. No wonder she had an affair. But why confess to it? Perhaps the excitement of her fantasies wore thin, and she became obsessed with the idea of confessing all, hence acquiring a thrill and notoriety of a different sort.

Gowdie speaks frequently of fairies, of meeting the king and queen of fairyland, and of them giving her “more meat than I could eat”.

Other magic turns up in Gowdie’s confessions, transformations in which the devil assumes many forms — a deer, a crow — and she herself turns into a cat or a hare.

On the nights of the sabbath, Gowdie would put a beanstalk or a piece of straw between her legs and say three times “Horse and hattock, horse and go, horse and pellattis, ho, ho!”, and off she would fly, wherever she wished.

She readily admitted to performing the black magic associated with witches. The account tells us she was repentant, which suggests she felt guilt about something. At one point she says: “I do not deserve to be seated here at ease and unharmed, but rather to be stretched on an iron rack: nor can my crimes be atoned for, were I to be drawn asunder by wild horses.”

Gowdie and the other witches in her coven performed acts of “white witchcraft”, such as easing fevers and healing broken limbs, but it’s the “black witchcraft” that stands out. She “blasted crops” and ruined soil. She robbed the grave of an unchristened baby and mutilated the corpse to use it in a magic potion. She raised the wind, soured milk, made people ill, and tried to kill the laird’s male children.

For Gowdie the most unforgivable acts involved shooting the “elf-arrows” the devil supplied her with at her neighbours. Gowdie made frequent raids upon the community. “What troubles me most,” she said, “is the killing of several people with the arrows I got from the devil.”

Did Gowdie and the others really kill these people? At this point it is likely we’ll never know. But why confess? She concealed her activities for 15 years, and could have kept up the pretence. Maybe she truly believed in her fantasy. Perhaps she thought confession would lead to salvation. Or, given her vivid imagination, it may show that she was mentally unbalanced.

Gowdie will remain something of a mystery — but the power to inspire artists three centuries after her death is magic in itself.

Prom 50, LSO, Monday, 7.30pm, BBC Radio 3; Gary Lachman’s Twenty Minutes — It Must Be Witchcraft is broadcast at 8.15pm

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