As our esteemed readers know, we at the Guardian fashion desk do not make a habit of blanket coverage every time a duchess wears a dress to a thing. There are issues that demand rolling coverage in the news right now, and common sense suggests that Kate’s decision to wear, say, a tweed suit versus a wool coatdress on a given day’s charity visit probably isn’t one of those issues. But when she wore a sparkling green Vampire’s Wife dress, with that label’s distinctively dramatic emo-mermaid silhouette and frilled bracelet-length sleeves, it was a genuine moment. Every fashion-based WhatsApp group started blinking in unison; the dress was headline news on Vogue soon after.

Accessorised with a pint of Guinness during the first stop on the Cambridges’ tour of Ireland, this was a coronation. Not for Kate – for the dress. It was the moment that the Vampire’s Wife dress became an official It dress. Not since Cameron Diaz and Scarlett Johansson simultaneously swooped on Roland Mouret’s Galaxy dress in 2005 has one dress been so ubiquitous. After a lengthy under-the-radar campaign of red-carpet appearances, the Falconetti, which costs £1,595, has branched out from its indie heartland to become a bona fide mainstream hit.

Facebook Twitter Pinterest Kate in the Falconetti dress by Vampire’s Wife. Photograph: REX/Shutterstock

Back in 2018, Rachel Weisz wore a floor-length, deepest aubergine version to the Baftas, Margot Robbie wore the same in crimson to a wedding in Australia and Jennifer Aniston chose an identical frock for a Jimmy Kimmel appearance. Last year, Kate Moss appeared in a Vampire’s Wife campaign modelling a shorter version, the Mayhem, while Princess Beatrice wore the Falconetti to Ellie Goulding’s wedding. This dress has crossed every divide: from London to LA, from music to fashion, and from alternative to establishment.

The Vampire’s Wife herself is the designer. The ex-model Susie Cave, 53, still has the milkmaid skin and Beetlejuice hair that made her a legendary beauty to whom Prince used to send red roses whenever he was in London. She launched her fashion label in 2014 with a name referencing her husband, the musician Nick Cave. Florence Welch, a devoted fan, has described the signature dress as “making you look like you are practising witchcraft in a very romantic cult”. The fabric is grand – corded dark velvet, or metallic silk chiffon in sweetie-wrapper bright – and the neckline is high and modest. The torso is body-skimming – but not tight – and ends in a seam positioned just above the natural waist. This simple shape is set off by narrow sleeves set on a posture-enhancing high armhole that flounce at the shoulder and waist. The skirt is softly gathered. The classic hemline is below the knee, although there is a short option beloved by Alexa Chung.

Facebook Twitter Pinterest The Vampire’s Wife designer, Susie Cave, with her husband, the musician Nick Cave. Photograph: David M Benett/Getty Images for mothers2mothers

The Vampire’s Wife phenomenon is remarkable for two reasons. First, famous women do not on the whole want to be seen in the same dress as everyone else, which means it takes a lot for a dress to overthrow the desire to look unique. Second, this dress is not an obvious choice. It is poetry, not pop. The elite few dresses that have enjoyed this degree of success have tended to be beloved for their figure-flattering properties. Mouret’s Galaxy dress, with its internal “power mesh” lining, was a double whammy of internal structure to suction and hold, with external styling that flattered (that elongating back zip) and added an air of sophistication (those origami-like folds at the neckline). Stella McCartney’s Octavia dress, which was everywhere in 2011 – Liv Tyler wore it, and Kate Winslet had it in two different colourways – used optical-illusion panels that functioned as a kind of hourglass-shaped filter over the body.

The Vampire’s Wife is different. The shape modestly covers the upper arms and thighs, but the silhouette is cut to work best on a narrow torso and flat stomach. (It is forgiving of age, however, because it covers a lot of skin.) This dress takes the wholesome, milk-fed aesthetic of the prairie dress – and its ubiquitous high-street offshoot, the midi-length ditsy floral dress that has a ruffle at the hem and is probably being worn by someone in your office or railway carriage right now, probably with flat white trainers or ankle boots – and gives it a cheekboned, hungry kind of glamour. Wearing a Vampire’s Wife dress is an elevated kind of a power move: not only are you showing (to those who know) that you have the body to look good in this dress, you are also signalling that you are sufficiently sophisticated in your thinking and messaging to be low-key about showing off.

Facebook Twitter Pinterest Cameron Diaz (left) and Scarlett Johansson (right) in Roland Mouret’s Galaxy dress; and Kate Winslet (centre) in Stella McCartney’s Octavia dress. Composite: Guardian Design Team

The story of the Vampire’s Wife label is an emotional one. The label launched in 2014 with a broad-based bohemian aesthetic that took in pastel-toned full skirts, silk blouses, cashmere cardigans and Liberty-print tailoring. Then in July 2015, Susie and Nick’s 15-year-old son, Arthur, fell to his death in a tragic accident close to the Brighton family home, and any thoughts of fashion disappeared from Susie’s universe. But three months later, prompted by friends trying to distract her from her grief on the pretext of needing dresses made, she found her way back to her design studio. “To be honest, work is what has saved me,” she told Vogue in 2017. It was during this highly charged time that she narrowed focus, distilling the aesthetic to its essence and hitting on the very particular silhouette with which it has become synonymous. This is a party dress that has an air of melancholy. It is a Disney princess dress for goths, a dancefloor dress for the era of Billie Eilish. It speaks not just of fashion sophistication, but of emotional sophistication.

This is a dress that speaks volumes. It is pretty, but it is slightly perplexing. And it is set to be everywhere in 2020: the most recent sighting was on Jodie Comer, while filming Killing Eve season three. The Duchess of Cambridge and Villanelle is about as strong a ticket as it gets. A dress that can unite those two is nothing short of magic. This spring, the Vampire’s Wife will have its moment in the sun.