The predatory partner of Helen Bailey has been found guilty of drugging and killing the children's author in a long-planned plot to acquire her riches.
Driven by greed, Ian Stewart secretly spent months poisoning the Electra Brown writer with his sedatives, smothering her once she had been stupefied in April last year.
The lifeless 51-year-old was dragged into a foul cesspit hidden deep below their luxury Hertfordshire home, where her body lay undiscovered for three months.
Following a six-week trial at St Albans Crown Court, a jury of five women and seven men found Stewart, 56, of Baldock Road, Royston, Hertfordshire, guilty of murder, fraud, three counts of perverting the course of justice and preventing a lawful burial.
Police said the "sudden, unexpected" death of Stewart's first wife, Diane - who was found in the couple's garden in 2010 - will now be re-examined.
Stewart scoped out the vulnerable widow on the internet in 2011 - earning his way into her trust and later her £3.3 million estate after launching a "love-bombing" offensive.
"She was being grossly deceived by someone who was preying on her," prosecutor Stuart Trimmer told his trial.
Over many weeks, Stewart surreptitiously fed Ms Bailey his prescription anti-insomnia drug, Zopiclone, possibly by slipping it into her morning scrambled eggs.
She soon became panicked by her deteriorating state of mind, searching online for terms such as "can't stop falling asleep" and expressing concern to loved ones.
A pillowcase found next to the author's body led the prosecution to suggest Stewart used a pillow to smother her while she was sedated.
Ms Bailey, known for her young adult stories and memoir on bereavement, was finally found submerged in a tank of human sewage underneath the couple's garage on July 15 last year.
Dead at her side was Boris the dog, her loyal companion.
Stewart, wearing a light blue top, remained emotionless in the dock as the verdicts were delivered by the jury foreman.
The author's shattered family welcomed the verdict, but said her death had left them lingering in a "long shadow of loss".
Ms Bailey's brother, John, sat in court for almost every day of her killer's trial, while their elderly mother Eileen broke down as she gave her testimony.
The family said in a statement: "Despite this victory for justice there can be no celebration.
"Our families have been devastated and nothing can ever bring Helen back to us, or truly right this wrong.
"A long shadow of loss has been cast over the lives of so many who will always remember Helen with enduring love and affection."
Stewart sparked a major search effort after feeding Ms Bailey's loved ones the lie that she had left abruptly to seek some "space" at her seaside cottage in Broadstairs, Kent.
Detective Chief Inspector Jerome Kent said: "To kill somebody was despicable enough, but to dispose of her in the way he did and lie to everyone including his own children shows how wicked and despicable that man is."
Stewart was branded a man who "lacks any remorse and empathy" by prosecutor Charles White.
He added: "He was an arch dissembler, he was able to trick everyone, so I think anybody who came across his path was a potential victim."
A financial inquiry will be held to ensure Stewart does not profit from the crime, Mr White said.
He will be sentenced on Thursday.
The jury returned with six guilty verdicts against Stewart after five-and-a-half hours of deliberation.
As Stewart was led from the dock, his eldest son, Jamie, did not meet his eye.
Outside the courtroom, tearful relatives of Ms Bailey embraced.
The couple met on a Facebook group for the bereaved and started a relationship within a year of Ms Bailey's first husband drowning on holiday.
Stewart soon became the chief heir to her fortune in a re-written will and gained power of attorney over her affairs.
He stood to gain around £1.8 million from her investment portfolio, plus the value of their home in Royston and her coastal cottage in Kent.
Hours after the murder, he also illicitly boosted a standing order to himself from her account.
This earned him an extra £12,000 over the three months following her disappearance, during which he played the part of a man wounded by his bride-to-be's abandonment.
The death of his first wife of apparently natural causes will now be probed by detectives involved in securing his conviction.
DCI Kent told the Press Association: "Diane Stewart died of natural causes in 2010, that is seven years ago.
"You will not be surprised that police investigating Ian Stewart for the murder of Helen Bailey would consider if there are any similar links to the death of his first wife.
"There is not a murder investigation into Diane Stewart, there is a re-examination of a sudden, unexpected death.
"There was a coroner's sudden death investigation, that would involve a post-mortem, histology samples being looked at and a pathologist determination - and the cause of death at that time was natural causes.
"It is only right that I would look back on somebody's past."
The former software engineer's deceptions culminated with a fantastical tale of a deadly kidnap plot by two mystery men called Nick and Joe.
From the witness box, he confessed lying about the author's whereabouts for eight months to keep his family safe.
Stewart will be sentenced by Judge Andrew Bright at 10am on Thursday.
Charles White, of the Crown Prosecution Service, said: "Ian Stewart murdered Helen Bailey and then conducted a cynical, deceitful and calculated charade as he watched the police conduct a futile missing person investigation.
"The prosecution presented a case to the jury which clearly showed how Stewart was linked to Helen's disappearance through mobile phone, computer and CCTV evidence.
"Our thoughts are with Helen's family and friends, who have been devastated by her death. I hope today's conviction reassures them that justice has been done."