The murder of a French nanny bears all the hallmarks of a psychosis known as folie a deux, where a delusion is shared from one individual to another, a court heard.
Sabrina Kouider, 35, and Ouissem Medouni, 40, were found guilty of killing Sophie Lionnet, 21, and burning her body in their garden.
Folie a deux, or "madness of two", was apparent in the case, according to the judge, with depressive and borderline personality Kouider the driving force and weak Medouni the willing party.
Kouider collapsed in tears as the jury foreman returned the verdicts, while Medouni hung his head.
Lionnet's mother Catherine Devallonne also wept as Judge Nicholas Hilliard QC said he was sure the allegations made against her daughter by her killers had "no truth whatsoever".
In the weeks leading up to her death in September last year, the couple beat, starved and tortured the shy 21-year-old au pair by dunking her head into water until she confessed to the supposed link to Mark Walton - a founding member of pop group Boyzone and Kouider's ex-boyfriend.
Having killed Lionnet in the bath, the pair threw her on a bonfire in the garden of their home near Wimbledon, south-west London, as they barbecued chicken nearby.
When firefighters were alerted by neighbours to pungent-smelling smoke, Medouni tried to pass off the charred remains as a sheep.
And Kouider claimed to police that Lionnet had run off with Walton. The defendants later admitted disposing of her body but denied Lionnet's murder, blaming each other for her death.
An Old Bailey jury found both of them guilty of the murder following a two-month trial that was described as stranger than fiction.
Lionnet's parents travelled from France to see the disturbing evidence as it unfolded.
The court heard how fashion designer Kouider was fixated with her ex-boyfriend Walton.
After splitting up after two years, Kouider reported him to police more than 30 times and received a caution for falsely branding him a paedophile on a fake Facebook profile.
She also accused him of sexually abusing a cat, using black magic and hiring a helicopter to spy on her.
Giving evidence, Los Angeles-based Walton said he had been "in love" with Kouider but she would "flip" and go "crazy" for no reason.
Another ex-boyfriend Anthony Francois described her as a "lunatic, fickle and unstable".
The mother-of-two created a fantasy world casting Walton as an evil villain who seduced Lionnet with sex and promises of Hollywood stardom.
Banker Medouni became an ardent believer in Kouider's twisted reality and they interrogated Lionnet for hours to get to "the truth".
Jurors heard more than eight hours of recordings in which Lionnet was slapped, likened to a Nazi collaborator and called "worse than a murderer" by her tormentors.
Kouider, who claimed to know influential people including US President Donald Trump, threatened to have her locked up and even marched her to a police station.
The victim's distraught mother Catherine Devallonne had begged Kouider to send her daughter home but she refused.
In her final days, Lionnet was hit with an electrical cable and beaten so badly she had five broken ribs and a cracked breast bone.
In a filmed "confession", the emaciated and broken young woman admitted she had drugged Medouni so Walton could sexually assault him. Within hours, she was dead.
According to Kouider, Medouni tortured her in the bath, then demanded they have sex as her dead body lay nearby.
She told jurors: "He was putting her head under the water and sometimes he would put water on the towel in her mouth. It was getting really mad."
Before the trial, Medouni claimed Lionnet died by accident after he punched her during an interrogation in the bath.
He offered to admit manslaughter but later retracted his confession, saying he made it to protect his wife, who has been diagnosed with a borderline personality disorder.
In his evidence, Medouni claimed his wife had woken him up in a state saying "what have I done, what have I done".
He was shocked to find Lionnet unconscious in the bath and tried to revive her, he claimed.
He said Kouider refused to call 999 and told him they would "burn her" instead.
A witness in the house, who cannot be identified for legal reasons, placed both defendants in the bathroom with Lionnet on the night of her death.
He described hearing Lionnet screaming and splashing in the bath as they said to "breathe".
Prosecutor Richard Horwell QC told jurors that neither were prepared to admit the truth - that they killed her out of "revenge and punishment".
He said their "unhealthy, myopic, all-consuming and groundless" obsession with Walton had deprived them of reason and turned their nanny into "something less than human".
The judge is expected to sentence the pair on June 26 at the Old Bailey.
Lionnet's mother Mrs Devallonne described her daughter's killers as "monsters".
She said: "These self-obsessed individuals who murdered Sophie did not believe Sophie had a value.
"These monsters repeatedly beat Sophie. They starved, tortured and broke her. They took away her dignity and finally her life.
"Our Sophie will soon be laid to rest. No God will ever forgive you both for what you have done to our daughter."
The victim's father Patrick Lionnet said: "Sabrina and Ouissem have not only stolen the life of my daughter so brutally and without remorse, they have also stolen mine."
He said what the couple did to his shy and reserved daughter was "beyond comprehension" and "unforgivable".
Aisling Hosein, a Senior Crown Prosecutor from the CPS London homicide team, said: "Only Kouider and Medouni know exactly how they killed Sophie but the prosecution was able to prove that she died as a result of purposeful and sustained violence, and not by accident.
"They were both jointly involved and came up with a plan to try and destroy her body and escape responsibility for this horrendous crime for which they have been found guilty.
"We thank the witnesses in the case along with our colleagues in France for their assistance which has led to these convictions. Our thoughts are with Sophie's family at this time."
Folie a deux - madness of two
The murder of Sophie Lionnet bears all the hallmarks of a psychosis known as folie a deux.
Folie a deux, or "madness of two", is defined as a delusion shared from one individual to another.
Depressive and borderline personality Sabrina Kouider was the driving force and weak Ouissem Medouni the willing party.
Even as the true facts were laid bare in court, the pair refused to accept what they had done was the product of their warped fantasy.
Judge Nicholas Hilliard QC said that even though there was evidence about their mental state, in particular Kouider, who was held at a medium secure mental hospital, none of it was a defence for murder.
Other high-profile and extreme examples of folie a deux include serial killers Fred and Rosemary West.
The case of New Zealand teenagers Pauline Parker and Juliet Hulme was made into the film Heavenly Creatures, starring Kate Winslett.
Perhaps the most strikingly similar case to be heard at the Old Bailey involved lesbian child killers Polly Chowdhury and Kiki Muddar in 2015.
The pair were convicted of the manslaughter of Chowdhury's eight-year-old daughter, Ayesha Ali, who died at their home in Chadwell Heath, east London, in August 2013.
Muddar, who like Kouider has a borderline personality disorder, convinced Chowdury her daughter was evil and together they punished and terrorised her with scary masks before she died in the bath.
Unlike Kouider and Medouni, Chowdhury accepted in the trial that she had been swept up Muddar's fantasy world of alter-egos.