Diddy watches as Cassie Ventura is sworn in as a witness. (Jane Rosenberg / Reuters)

Casandra Ventura, the R&B singer and model whose explosive civil lawsuit set the stage for this federal trial, took the stand today. Ventura, visibly pregnant and soft-spoken, testified in wrenching and graphic terms about the alleged abuse during her 10-year on-and-off relationship with Diddy, who has pleaded not guilty to sex trafficking and racketeering charges.

Ventura, who is expected to testify for days, told jurors about “violent arguments” that escalated into ferocious attacks. “He would mash my head, knock me over, drag me, kick me,” Ventura told the panel of eight men and four women. She characterized Diddy as a mercurial and sometimes “scary” figure who “controlled a lot of my life.”

She described Diddy’s constant demand for drug-fueled marathon sexual encounters known as “freak offs.” She said that her “stomach churned” the first time he asked her to have sex with another man while he watched. In the years after, “the freak offs became a job, and there was no space to do anything else. … They would be 36, 48, 72 hours. The longest one was four days.”

The “freak offs” left her feeling “disgusting” and “humiliated,” she said, fighting back tears. She said she took drugs like ecstasy to create a “buffer” so she could “disassociate.” Diddy seemed to become another person altogether during the highly choreographed sex sessions. “His eyes would go black,” she said. “The version I knew of him was no longer there.”

Here’s what else you need to know:

— Daniel Arkin, national reporter

The courtroom was gripped by Ventura’s presence from the minute she walked in, wearing a form-fitting, long-sleeved brown dress that showed off her pregnancy. Diddy leaned back in his seat and watched her intently as she recounted years of alleged abuse at his hands in stark, explicit language. She frequently took deep breaths or dabbed her eyes with a tissue while answering questions. She wept as she talked about the emotional toll of trying to keep Diddy happy and avoid one of his rages.

Diddy, for his part, arrived this morning in what appeared to be the same outfit he wore yesterday: a cream-colored sweater, white button-down shirt and slacks. Diddy’s family — including his mother, Janice Combs — showed up in court again today. At one point, Diddy looked back at his relatives and made a heart symbol with his hands.

Ventura’s testimony about “freak offs” and Diddy’s alleged sexual abuse was both dramatic and scandalous — but her testimony on other topics is likely more crucial for the government’s case.

Ventura also testified about the hiring of an escort and “performing” at Diddy’s direction. The escort testimony is key to the government’s sex trafficking and interstate prostitution charges, and it’s important for the racketeering charge too. In fact, a lot of Ventura’s testimony is just devastating for Diddy — but it’s not as critical to the elements of the alleged crimes as other testimony from other witnesses.

For example, to prove that Diddy violated the Mann Act, the government can meet its burden by calling an unnamed escort to testify he was paid for sex, and that interstate travel was involved. The Mann Act charge requires interstate travel plus prostitution — and that’s it. In that sense, Ventura may be the prosecution’s star witness, but she is probably not the most essential for its case.

Tomorrow: Ventura is expected to return to the stand. It’s likely she will continue to testify about Diddy attacking her in the hallway of a Los Angeles hotel in 2016. NBC News’ team will live-blog everything that happens.

PSA: Every night during Diddy’s trial, NBC’s “Dateline” will drop special episodes of the “True Crime Weekly” podcast to get you up to speed on the case. “Dateline” correspondent Andrea Canning will be in conversation with NBC News’ Chloe Melas and special guests — right in front of the courthouse. Listen here. 🎧

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