She once wielded so much influence the King of England himself gave up the throne just to be able to spend the rest of his life with her.

But in her final years, Wallis Simpson was a tragic shadow of her former self. 

The glamorous American divorcee, around whom a constitutional crisis had been triggered when Edward VIII chose love over duty, was reduced to a frail figure ravaged by dementia and poor health, as well as loneliness after the death of her beloved husband years earlier.

But she also fell victim to the cruel machinations of her French lawyer, Suzanne Blum, who financially abused and isolated the Duchess in the waning years of her life.

Insiders say she kept Wallis away from those closest to her, sold her heirlooms for her own profit and assumed power of attorney over her estate.  

Now the coercive relationship is set to be played out on screen in new biopic The Bitter End starring Hollywood icons Joan Collins as the Duchess and Isabella Rossellini as Blum.

But just how much control did Blum exert over the ailing aristocrat?

According to Royal biographer Hugo Vicklers, she was a 'Satanic figure … wearing the mantle of good intention to disguise her inner malevolence'.

The dynamic between the two women was one seen first-hand by Johanna Schutz, employed by the Windsors as their private secretary in 1969, and who became 'like a daughter' to the couple.

Schutz - who only spoke for the first time about her experiences in 2020 - was a young 26-year-old when she was recruited into their household.

She recalled how every time the Duchess would leave their home, Villa Windsor, in Paris, devoted Edward would wait for her as she came down the stairs before escorting her to the front door - and he would be again waiting for her when she returned.

On occasion, Wallis intimated that she felt 'trapped' by his all-consuming love, but nevertheless the couple remained dedicated to one another for 35 years.

When he died in 1972, aged 77, the Duchess was bereft.

Schutz stepped in and rarely left her side, ensuring she ate every meal with the grief-stricken widow and regularly accompanied her by boat to her native America. 

She recalled: 'I couldn't replace the Duke but I could support the Duchess, which was a pleasure.'

Schutz was in many ways one of the only people Simpson formed a close relationship with. She had no children with the Duke and the Royal family had largely shunned her for her role in Edward's abdication in 1936. 

He made the decision to give up the throne after being told in no uncertain terms by Stanley Baldwin's government that he would not be allowed to marry a divorced woman and remain as King.

His departure forced his younger brother, the Duke of York, to step up and become King George VI, creating a family wound that never healed.

Wallis was blamed by royals such as the Queen Mother - George VI's wife - for tearing the family apart, with relations souring further by the Duchess' nickname for her sister-in-law - 'Cookie', a disrespectful reference to her weight.

Once free from the weight of responsibility, the Duke and Duchess of Windsor led the life of the idle rich, attending parties in France and America and going on lavish holidays with close friends.

Though she lacked the status of an actual queen, it was the kind of high life that American had always sought.

But after her husband died, her fortunes rapidly changed.    

Because he had lived largely beyond his means, with his only major source of income coming from a royal allowance he always complained was too low, the Duke of Windsor left his wife obliged to cut back on the luxurious living to which she had become accustomed.

The French government kindly agreed to defer death duties, while the City of Paris allowed Wallis to live in the Bois de Boulogne home that she and her husband had shared since their marriage at a moderate rent.  

She could still, at least, have a comfortable existence with some dignity. 

But it was that very home that would eventually become a prison, in no small part thanks to the cruelty of Blum. 

What had started out as a business relationship - with Wallis consulting her as and when needed - turned into one of abuse and control.

Historian Hugo Vickers told in his 2011 book how, step by step, Blum dismissed the Duchess's English lawyer and then her staff, who had included a chef, concierge, chauffer and hairdresser.

She gradually banished friends who wanted to come and visit, claiming that the Duchess was too tired or would be too upset to see them.

Her health then declined and she was left in a state of immobility.

A diagnosis of atherosclerosis - where the arteries become narrowed - led to periods of confusion, prompting Wallis to believe her husband was still alive. 

She would imagine herself back at her the worst point in her life, when Edward VIII was about to abdicate. 

Then in 1972, the Duchess fell out of bed over Christmas and was not given appropriate treatment, despite being in considerable pain. 

It only emerged months later that she had broken her hip. The then 76-year-old needed surgery, and while she was in hospital Blum dismissed lawyer Godfrey Morley - who had previously handled the Duke's affairs - after persuading Wallis he was trying to get his hands on her money. 

A letter signed soon after by the Duchess appointed Blum as her sole legal representative. 

The lawyer was then advanced in the Legion d'Honneur, France's highest decoration, after the Duchess changed her will so that many of her possessions would be left to the country's great museums.

The move was a gesture of gratitude to the authorities for providing her home at a peppercorn rent.

In November 1975, four months before her 80th birthday, the Duchess was struck down by a huge intestinal haemorrhage, and it was this severe downturn in her health that Blum used to her advantage. 

The Duchess had returned from hospital a virtual wreck, unable to move and then later could not even speak.

She would plead with nurses when in pain, hoping that 'the Good Lord would take her away'.

As Vickers recounted, one of the nurses said: 'It gives me great distress to see HRH, who was once a great lady, admired and feted throughout the world, who showed courage which was widely respected, becoming little by little a lady who suffers terribly.' 

In January 1976, having instilled panic in the Duchess about overspending, Blum announced she had been given instructions about what silver and porcelain objects needed to be sold.

While the Duchess initially refused to sign letters authorising the sales, the objects were still distributed.

Swiss banker Maurice Amiguet was given earrings, a bracelet and a necklace, whilst the Duchess's doctor, Jean Thin, was handed watches and gold box. 

Blum herself received jewels including a ring adorned with an oval amethyst and diamonds and a Louis XV gold box.

She later took objects including a pair of ruby earrings, a gold Cartier watch and a gold cigarette box inscribed 'David from Wallis 1935 Christmas'. 

They were allegedly set to be handed out as gifts. 

The distribution of her personal effects flew in the face of the Duke's will, which had stated that once the Duchess had died all of their possessions - money, jewellery, paintings and artefacts - should be returned to the Royal family. 

Schutz recalled: 'Blum really threatened the Duchess. She told her that the French government would make her leave the house (where the Windsors lived rent- and tax-free) unless she bequeathed everything to the Louis Pasteur Institute. She was totally menacing.'

Defying the Duke's wishes, after Wallis died in 1986, her whole collection was sold at Sotheby's for £31million, with the proceeds going to the Pasteur Institute. 

In another incident, the Windsors' butler, George, came to Schutz with a box filled with letters between the couple. He had been told by the Duchess to burn them, but Schultz recognised their historic value. 

She attempted to keep them safe, but after the Duchess was dead, Blum had them published. That was something Wallis would 'never' have wanted, Schultz said. 

Her concerns for Wallis deepened. She claimed nurses hired by Blum began 'drugging' the elderly lady, meaning she was largely unaware as her lawyer sold off all of her possessions. 

Schutz finally left in 1978, refusing to sign a new contract that would have meant she was working for Blum directly, not the Duchess. 

When Wallis could no longer recognise her, she made the 'heart-breaking' decision to leave her employment. 

Blum meanwhile continued her cruel treatment. 

In an act that would have further deepened her unhappiness, Wallis's beloved pugs, Ginseng and Diamond, were taken away from her over fears they might infect her. The Duchess never saw them again.  

Her night nurse, Elvire Gozin, who continued tending her until her death, later told how she 'died in a slum' and had become a 'prisoner in her own home'.

Hairdresser visits had been terminated and expensive creams from Estee Lauder replaced with cheap make-up, whilst bedclothes became tattered. 

Although Gozin twice attempted to alert the Queen to the Duchess's plight, she was never able to get access to her or pass on the message.

Gozin took photos of the Duchess in her bed which were published after her death. 

They showed her lying amidst the machinery that was keeping her alive, with her head just visible above the sheets. 

And, in what was a further indication of her life of utter misery, Dr Thin told a newspaper how he had ordered the Duchess's wedding ring to be 'gently cut off' because of her severe arthritis. 

During these last years and months, one consistent visitor was the Right Reverend James Leo, the Dean of the American Cathedral in Paris.

It was he who performed the last rites in April 1986. He said: 'She squeezed my hand during the last rites and again as I read a short passage from the Bible.'  

When she did finally pass away, her close friend Lady Diana Mosley said her final years were 'not really a life at all.'

'I'm delighted to hear she has died. I wish she'd died many years ago,' she added.

Her funeral service at St George's Chapel lasted for less than half an hour and was stripped of nearly all the pomp and ceremony that usually marks a Royal passing.

Other members of the 100-strong guest list included the then Prince Charles and his wife Princess Diana, along with Prime Minister Margaret Thatcher and Labour leader Neil Kinnock.

On her coffin was a single wreath of white, orange and yellow lillies left by the Queen.  

Her burial next to her husband outside Frogmore Mausoleum was attended by only the Queen, the Duke of Edinburgh, Prince Charles and Princess Diana and eight of Wallis's aides and friends.

The Queen Mother, who had once called Wallis the 'lowest of the low', did not attend the burial, after being asked to stay away by the Queen.

However, there was one alleged flicker of emotion that perhaps signalled how, despite all that happened, feelings could be complicated. 

According to Princess Diana, the Queen did shed a tear as the Duchess was laid to rest. She claimed it was the only time she had seen the monarch weep.

The laying to rest of the Duchess of Windsor not only marked the final chapter in a marriage that captivated and scandalised in equal measure, but also brought to a close 15 years of abuse that Wallis endured at the hands of the predatory Blum.