Convicted double killer Colin Howell has said his former lover entered into a "blood pact" with him at the time she had a secret abortion in London.
Howell, 51, has been giving evidence for a second day at the trial of Hazel Stewart, 47.
She denies being part of a joint venture to kill his former wife and her first husband in May 1991.
Howell told Coleraine Crown Court the abortion had strengthened their bond during their affair.
"It was like a blood contract which we had secretly signed together of the murder of an unborn baby," he said.
"It was a huge bond. It did not make it better."
Mrs Stewart denies murdering Trevor Buchanan, 32, and Lesley Howell, 31, whose bodies were found in a fume-filled car in Castlerock.
Howell, a former dentist, has already pleaded guilty to the charges and was jailed for 21 years.
He said he and his first wife Lesley had three abortions in the year before they were married, two of them inside the space of seven months in 1982.
Howell said the bond with Mrs Stewart was an "unseen and powerful" one which could only be broken through confession and seeking forgiveness.
He claimed in court on Tuesday he was within hours of owning up to police in 1998 about the murders of Trevor Buchanan and Lesley Howell.
Howell said that after first revealing his guilt to his second wife Kyle, two years after they married, he made arrangements to bring the two families together at a hotel in Newcastle, County Down, make his confession and then hand himself over to the authorities.
He said his second wife was shocked by his revelations and told him he had "to go to the police".
However, he told the jury that he underwent a "religious conviction" after meeting a girl at a Sunday night church service where he claimed she told him his sins had "been forgiven and forgotten by God".
He said he and his wife then decided to keep his guilt secret for the sake of their children.
Howell told the court on Tuesday he and Mrs Stewart had been "waltzing in time" and she had always been in perfect harmony with him.
He referred to himself as the "mastermind" of the murder plot.
The defence asked Howell if it was true he had controlled Mrs Stewart.
The barrister said Howell had agreed during police interviews in 2009 that Mrs Stewart was frightened of him that she was "kind and innocent" and easily led.
Howell said he had been agreeing with everything police had said, treating officers like church elders.
He added: "If I had been accused of killing JR Ewing or JFK I would have said that."
He said it later dawned on him that Mrs Stewart was setting herself up to be a victim, but she had never objected to the murder plan.
Howell, said the last thing he wanted to do was "make things worse for Hazel".
But three times he said he and the accused had been "waltzing in time", explaining that he had been the lead, but she had always been in perfect harmony with him and he did not "drag her along the floor".
Howell said it was only two years ago - when he confessed to police about the murders - that he had realised Hazel Stewart was not his responsibility.
He denied he was psychotic - the assessment of specialist psychiatrist Dr Helen Harbinson who examined him in prison.
"I don't agree with the conclusion of Helen Harbinson or any of the other psychiatrists," he said.
"I believe any human being has potential to do what I did, but I did it - that's what sets me apart from most of humanity."
Dr Harbinson had said Howell had drugged Mrs Stewart before they had sex as an experiment to see if he could do the same with his patients,
After her report was quoted by Mrs Stewart's defence lawyer in court, Howell admitted to drugging the mother-of-two during their six-year relationship and to sexually assaulting three patients in his practice in Ballymoney, County Antrim, as they emerged from sedation.
But he denied he used laughing gas and, on one occasion, injected anaesthetic on Mrs Stewart prior to sex to find out what he could do to patients.
Howell, who also said he had a nine month sexual relationship with a staff member at his practice during his second marriage in 2005, pleaded guilty in court to indecently assaulting three women patients last year.
However, he claimed he used the drugs on his former lover to help her assuage the guilt she experienced having sex with him after the double murder of her husband and his wife.