Chris Watts made global headlines in 2018 when he admitted to brutally murdering his entire family.
The 34-year-old Colorado man is serving five life sentences in a maximum-security prison in Wisconsin after he strangled and smothered his wife Shanann, 34, who was carrying his unborn son, Nico at the time, and two daughters, Bella, four, and Celeste, three.
As the killer approaches his 35th birthday on May 16, he has had to face the fact he will be behind bars for the rest of his life.
For 23 hours a day, Watts is on lockdown in his cell, according to People magazine, where he can only leave the confined space for exercise and showers.
"Nothing changes," a source who spoke to Watts in prison told the publication, according to news.com.au.
"Every day is like the day before, and every day in his future will be the same as today."
Last November, People reported Watts had found religion in prison, and that he reads his Bible, reflecting on his sins.
"A day doesn't go by – a minute doesn't go by – where he doesn't think about his family," a source said. "He's in a hell of his own making."
In August 2018, Watts strangled Shanann and buried her body in a shallow grave.
He also smothered both his young daughters and dumped their bodies in a separate oil tanks on a remote oilfield owned by his employer at the time, Anadarko.
In an interview with author Cheryln Cadle for her new book Letters From Christopher, Watts explained that had he not met his mistress Nikki Kessinger, "I would never have killed my family."
In an interview with the Daily Mail, Cadle said Watts was "mesmerised" by his mistress.
"She showed him respect that he didn't feel like he'd ever been shown before," Cadle said.
In the book, she claims Watts clearly remembered the look on Shanann's face as he strangled her.
"Isn't it weird how I look back and what I remember so much is her face getting all black with streaks of mascara?" Watts told the author.
"I knew if I took my hands off of her, she would still keep me from Nikki. They asked me why she couldn't fight back; it's because she couldn't fight back."
He had been seeing her for two months before carrying out the horrific murders.
Watts was so enamoured with Ms Kessinger when she told him she wanted to "give him his first son" he allegedly tried to cause his wife to miscarry by giving her a strong painkiller.
"Christopher says he loved her like he has never loved anyone else before," Cadle told the Daily Mail. "At the same time, he loves Shanann, although he knows that she was not his soulmate and not the person he was supposed to be with."
He told the author he felt "free" to be with Ms Kessinger and felt "no remorse" after the heinous crime.
After killing his wife, Watts claimed Bella saw him smother Celeste and screamed, "Daddy, no!" before he killed her.
Eerily and inexplicably, Watts admitted to investigators he mourns his dead wife and daughters daily.
He has pictures of them in his prison cell and claimed he talks to them every morning and night.
At first, Watts denied any wrongdoing, even going on live TV shortly after burying their bodies, begging anyone with information to come forward.
But investigators soon found the father-of-two was behind the brutal murders.
Apart from his emotionless behaviour during his first news interview, investigators said red flags were raised in how Watts referred to his family.
He kept describing Shanann and his two little girls in the past tense.
Not only did Watts say Celeste "loved" a certain pair of shoes, but he said his now deceased four-year-old "was a girly girl", People magazine reported.
"If he is hoping and believing that they're still alive, so why would you start talking about them in past tense?" Colorado Bureau of Investigation (CBI) agent Tammy Lee said in a new documentary, Criminal Confessions.
A lie-detector test also exposed Watts' affair with his co-worker Ms Kessinger, something he had previously denied.
"I hope that, you know, if you did have something to do with their disappearance … it would be really stupid for you to come in and take a polygraph today," Ms Lee said to Watts before the test.
After a line of questions, it was ruled that Watts had failed the polygraph test, and he later confessed to having an affair.
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But investigators needed more. They needed him to confess he was behind his family's deaths, and that's when they began to paint Shanann as the villain, a technique Ms Lee said investigators used to eventually get the truth.
Watts was sentenced to life in prison for the August 2018 murders of his family.
He was also forced to pay $US6 million ($A8.85 million) to his wife's parents to prevent him from profiting from the murders should he ever decide to write a book or sell the rights to his story.