The women were the last two remaining in the park after an outdoor gathering to celebrate Ms Henry’s 46th birthday when they were ambushed by Hussein.
The killer stabbed the two sisters repeatedly before dragging their bodies into the bushes, leaving them together as he fled the park.
Police discovered Hussein had signed his name in blood on a self-penned pact with a demon, promising to “perform a minimum of six sacrifices every six months for as long as I am free and physically capable”.
Hussein pledged to only target women and “build a temple” for the demon, and in return he believed he would win the National Lottery Mega Millions Super Jackpot.
At the Old Bailey on Tuesday, a jury found Hussein guilty of two counts of murder as well as possession of a knife.
He will now face a mandatory life sentence for the two killings.
In a victim impact statement read to court, the sisters’ mother Mina Smallman said: “No-one expects their children to die before them but to have two of your three children murdered overnight is just incomprehensible.”
She said that losing her two girls was “enough to shake a person’s faith – fortunately it did not”.
She said that before the trial, she had hoped the defendant would “do the right thing, a kindness and find some humanity in him”.
Listening to the detail of how her daughters died in court was “horrific” and made her think “this person cannot have a heart”, she said.
She added: “If any good comes out of this at least another four women will not meet a similar end in a so-called pact with a so-called demon.”
Bibaa Henry’s father Herman Henry described the “agony” of his loss, saying: “She was a beautiful outgoing strong minded and intelligent woman.”
He said nothing would ever bring back his daughter and her half sister Nicole, adding that Bibaa’s birthday would be a “constant reminder” of what happened.
Heartbreaking pictures of the sisters’ final hours together were revealed during the trial, as they enjoyed a sunny afternoon and evening in the company of friends.
Nina Esmat was one of the friends at the picnic, laid out with blankets, cushions and fairy lights as they reunited following the first Covid-19 lockdown. She was among the last to leave the event, at around 11.45pm, telling the court: “It felt a safe space.”
Ms Smallman’s boyfriend Adam Stone had received her last text message, at 1am on June 6, saying: “I’m dancing in a field.”
When the sisters failed to return home from the party, worried family and friends reported them missing to police and organised a search of the country park.
Ms Esmat said she spotted Ms Henry’s new designer prescription sunglasses in the grass.
“My heart sank”, she said. “I just knew she would not have left them behind.”
It was Mr Stone who made the horrific discovery of the two bodies, lying together in a hedgerow.
“I fell to my knees in front of Nicole”, he said.
“I screamed and I saw my mum and dad were a couple of metres behind me.
“I jumped up and turned around, gave my phone to my dad.
“I was still connected to 999 and grabbed them by the shoulders and told them not to look in there.”
Hussein had pleaded not guilty to the charges, but did not give evidence at his trial.
When arrested, he tried to explain away injuries by claiming he had been robbed the previous night. But Hussein faced a mass of evidence that identified him as the killer.
He had gone on to buy lottery tickets after the murders, which did not scoop him the jackpot but were found with his handwritten demonic pact.
The teenager, who had only just turned 18 when he killed Ms Henry and Ms Smallman, had been caught on CCTV buying knives, while DNA linked him to the crime scene.
In his pact, Hussein had believed he would “receive fruitful rewards in return for the future sacrifices I make to you, the rewards could consist of wealth and power”.
And he wanted “to never be suspected of any crimes by the police, and also that the police will never know of any crimes that I have done and that I will do.”
Following the guilty verdicts, Olcay Sapanoglu from the CPS said: “Nicole Smallman and Bibaa Henry were loving sisters who adored each other and had a real zest for life. But their lives were unexpectedly and cruelly cut short in the most horrific of ways as they celebrated a birthday together.
“Danyal Hussein targeted them – two women in the dark. His actions were part of an absurd and twisted deal he thought he was making with a demon – and he executed his part of the deal with shocking savagery. His victims stood no chance. He had come prepared to ‘sacrifice’ women.
“The prosecution case included CCTV evidence and strong scientific analysis of DNA and blood found at the scene which linked to Hussein.
“Hussein has shown no remorse throughout these proceedings. He has consistently denied being the person caught on CCTV making his way to and from the park or being the person buying the murder weapon at an Asda supermarket.
“The deaths of Ms Henry and Ms Smallman have devastated their loved ones and left a local community reeling. Our thoughts remain with all those affected.”
Hussein has been remanded in custody until sentencing on September 22.