Alleged "doomsday" triple murderer Chad Daybell's "desire for sex, money and power" is what drove him to kill his first wife and two stepchildren, prosecutors said in scathing opening statements on day one of his high-profile murder trial.

Chad, 55, has been charged with first-degree murder, insurance fraud, conspiracy to commit murder and grand theft, in connection with the 2019 deaths of 49-year-old Tammy Daybell, 7-year-old JJ Vallow and 16-year-old Tylee Ryan. 

He pleaded not guilty to the charges.

While investigators initially believed Tammy's death was natural, a medical examiner later determined she died by asphyxiation. JJ also died by asphyxiation, while Tylee's death was ruled homicide by unspecified means, according to the medical examiner.

On Wednesday, prosecutors laid out the motive behind the three merciless slayings.

"Two dead children buried in the defendant Chad Daybell's backyard in September of 2018," prosecutor Rob Wood told the jury, according to CNN.

"The next month his wife is found dead in their marital bed. Seventeen days after the death of his wife, Tammy Daybell, this defendant is photographed laughing and dancing on a beach in Hawaii at his wedding to Lori Vallow, a woman who was his mistress and the mother of the children buried in the graves on his property. Three dead bodies," said Wood.

"His desire for sex, money and power led him to pursue those ambitions," Wood alleged. "And this pursuit led to the deaths of his wife and Lori's two innocent children."

Chad and Lori met at a religious conference in Utah in 2018 and began a whirlwind romance, bonding over their "doomsday" obsession while Chad was still married to Tammy.

The couple's alleged aligned belief JJ and Tylee were "dark spirits," "zombies," and "obstacles" was another driver behind the killings, said prosecutors, according to ABC News. 

"The evidence will show that this was a convenient narrative," Wood told jurors, CNN reported. "This narrative gave them the pretext to remove people from this world for their own good."

Chad – described by the prosecution as a "seemingly ordinary man" who wrote books on the apocalypse – allegedly buried the kids on his Rexburg, Idaho, property, prosecutors said. Their remains were recovered in 2020.

JJ was found with his mouth duct taped and a plastic bag over his head, while Tylee was dismembered.

In May, Lori was convicted of murder and grand theft in connection with the deaths of her children. She was sentenced to life in prison without the possibility of parole. 

Her lawyers have since appealed her convictions. 

Chad's trial is expected to last two months.