CHICAGO (CBS) — The case against a former Northwestern University professor accused of murder just got a lot stronger.

As CBS 2’s Charlie De Mar reported Monday evening, Wyndam Lathem’s co-defendant has agreed to testify against him.

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Lathem and Oxford University employee Andrew Warren are both charged in the savage stabbing death of Lathem’s boyfriend, 26-year-old Trenton Cornell-Duranleau, in 2017.

Trenton James Cornell-Duranleau (Family Photo)

Prosecutors allege the murder was carried out to fulfill a “sexual fantasy.”

Warren was supposed to go on trial next week, but on Monday his attorneys announced he was pleading guilty and agreeing to testify on behalf of the prosecution.

Warren is expected to be sentenced to 45 years in prison.

Andrew Warren

Prosecutors said Lathem and Warren spent months planning to commit multiple murders, then to kill each other. On July 27, 2017, the pair teamed up to kill Cornell-Duranleau inside Lathem’s apartment in the River North neighborhood, prosecutors alleged.

“The victim was stabbed a total of 70 times,” said Cook County Assistant State’s Attorney Natosha Toller.

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The stabbings were of such force that one of the knives broke, prosecutors alleged.

Warren was supposed to videotape Lathem stabbing Cornell-Duranleau as he slept, but the victim woke up and began to fight back, prompting Lathem to call out to Warren for help, prosecutors said.

After killing Cornell-Duranleau, Lathem and Warren allegedly fled to Wisconsin, surprising some by stopping at a library in Lake Geneva and making a donation in Cornell-Duranleau’s name.

The duo was eventually arrested in California.

Wyndham Lathem

Prosecutors said before Lathem was apprehended, he sent a video to family and friends saying the murder was not an accident and that he is not the person people thought he was.

Lathem’s attorney released a statement in relation to the latest developments.

“Guilty people often plead guilty,” the statement said. “Our client persists in his plea of not guilty.”

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Lathem will be back in court on Tuesday, where the plea deal might be unsealed.