The star witness of the federal sex-trafficking trial of the music mogul Sean Combs took the stand for the first time on Tuesday: his former girlfriend Casandra Ventura, the singer known as Cassie, who filed a civil suit in 2023 that accused him of rape and years of physical abuse.

Ms. Ventura’s suit, which was quickly settled, was followed by a criminal investigation that led to the trial in Federal District Court in Lower Manhattan. Parts of the indictment against Mr. Combs, who is also known as Diddy and Puff Daddy, closely mirror the details and language of the lawsuit.

Hotel video: Israel Florez, a security guard at the InterContinental Hotel in Los Angeles, told jurors on Monday that Mr. Combs was caught on security cameras striking, hitting and kicking Ms. Ventura in a hallway in 2016. Mr. Florez said that Ms. Ventura had a “purple eye,” and that Mr. Combs offered him cash.

The jurors: Eight men and four women, who range in age from their 30s to their 70s, were sworn in as anonymous jurors. They will not be sequestered so must shield themselves from the media coverage about the case.

Sean Combs’s moods were extreme, Casandra Ventura says. He would object if she was being a “brat.” She adds: “Make the wrong face, and the next thing I knew, I was getting hit in the face.”

Casandra Ventura is explaining the complexity of her relationship with Sean Combs. At times, she is describing excitement, attraction, a whirlwind. Their romance began on a trip to Miami, where they had sex on a boat and he gave her a “blue dolphin” Ecstasy pill. But when she didn’t answer his phone calls, she says, he could call her “incessantly” or have his assistants and security personnel track her down.

Casandra Ventura is describing life with Sean Combs once she started dating him. “This lifestyle was much different,” she says. “He had assistants at his beck and call. He could get anything done quickly. He had respect from everyone and he traveled quite a bit.”

At the beginning of her relationship with Combs, she says, “I think I was just enamored by him. We were just having a good time.”

Ventura was asked why she wanted to be around Sean Combs. “I wanted to be around Sean for the same reasons as everyone else at the time,” she testified. “He was just this exciting, entertaining, fun guy that also happened to have, you know, my career in his hands.”

Ventura was asked what she knew of Sean Combs when she met him around 2005, when she was 19. “I just knew that he was this larger-than-life entrepreneur, musician. Was a fan of the music. I didn’t know too much about him personally.”

Soon after, she said, she signed a 10-album deal with Combs’s label, Bad Boy.

Casandra Ventura is speaking softly on the stand, and lifting a tissue to dab at her nose and eyes.

Ventura is visbily pregnant — eight and a half months, Sean Combs’s lawyers said. The prosecutors offered her water, and she is expected to take breaks in what could be four days of testimony.

Sean Combs ultimately “controlled a lot of my life,” Ventura says. Over time, she says, the videos he shot of her during sexual encounters were “blackmail materials,” which she feared could be put on the internet. “He had many resources to do that.”

Ventura is describing the complex nature of her relationship with Sean Combs, from their first “freak-offs.” She says those events caused her “nervousness and confusion.” She says she didn’t understand how these encounters could be a turn-on, but she felt a “responsibility” to please Combs. “I was confused, nervous, but also loved him very much,” she says.

Casandra Ventura, the singer and model known as Cassie, began testifying for a federal jury on Tuesday morning in the sex-trafficking and racketeering case against Sean Combs. Lawyers for Mr. Combs have portrayed the relationship as loving but deeply toxic while maintaining that any sexual arrangements were completely consensual.

In the first minutes of her testimony, she was asked by prosecutors to describe the more than decade-long relationship she had with Mr. Combs.

“There were violent arguments that would usually result in some sort of physical abuse,” she answered. “Dragging, different things of that nature.”

Ms. Ventura, 38, wore a brown turtleneck dress that accentuated her pregnant belly. As she entered court Mr. Combs turned back in his chair to see her walk in. His lawyers had asked the judge to have her present on the stand before the jury entered, a request that the judge apparently denied.

In her testimony, which is expected to last through the end of the week, Ms. Ventura was soft-spoken and visibly emotional, dabbing at her nose and eyes with a tissue. She recounted meeting Mr. Combs around 2005, when she was 19.

“He was this larger-than-life entrepreneur, musician,” she said. “Was a fan of the music. I didn’t know too much about him personally.” Soon after, she signed a 10-album deal with his label, Bad Boy. At that time, she said, she and Mr. Combs still had a “platonic” relationship.

“I wanted to be around Sean for the same reasons as everyone else at the time,” she testified. “He was just this exciting, entertaining, fun guy that also happened to have, you know, my career in his hands.”

Their relationship soon became sexual. Ms. Ventura described her first experiences with “freak-offs,” the drug-fueled sexual encounters with male prostitutes that the government contends were coerced. She said those events caused her “nervousness and confusion.” She did not understand how they could be a turn-on, she said, but felt a “responsibility” to please Mr. Combs. “I was confused, nervous, but also loved him very much,” she said.

Later she testified, “Eventually it became a job for me, pretty much.”

Her husband, Alex Fine, was allowed to be present in the courtroom for the beginning of her testimony, but the judge said Mr. Fine would have to leave during a discussion about an allegation that Mr. Combs had raped her in 2018. Mr. Combs’s lawyers argued that they might need to call Mr. Fine as a witness later.

The much-anticipated testimony was Ms. Ventura’s first major public comment since she filed a bombshell lawsuit against Mr. Combs, her former boyfriend and label boss, in late 2023, in which she accused him of having instituted a system of abuse and control over her life and career for more than a decade. Combs and Ventura quickly reached an eight-figure settlement in the civil case, which led to a government investigation and Mr. Combs’s arrest in September 2024.

Though legal filings in the federal case had referred to her only as Victim-1, there was never much doubt that the singer was the witness at the center of the racketeering conspiracy and sex-trafficking case against him.

On the stand, Ms. Ventura said that Mr. Combs ultimately “controlled a lot of my life.” Over time, she said, the videos he shot of her during the freak-offs were “blackmail materials,” which she feared could be put on the internet.

“He had many resources to do that,” she said.

Ventura is being asked by prosecutors to describe what a “freak-off” was. “It basically entails the hiring of an escort and setting up this experience so that I could perform for Sean,” she says, in halting speech. It involved Combs “being able to watch me with the other person and actually direct us on what we were doing,” she says. “Eventually it became a job for me, pretty much.”

Ventura is asked to describe her relationship with Sean Combs. “There were violent arguments that would usually result in some sort of physical abuse. Dragging, different things of that nature,” she says.

Ventura just walked into the courtroom with the jury present. She is pregnant — in her third trimester. She is the epicenter of this entire case. Without her lawsuit in November 2023 that accused Sean Combs of physical and sexual abuse, it’s likely none of this would be happening.

Combs turned back in his chair to see her walk in.

The defense asked that Casandra Ventura’s husband, Alex Fine, be barred from watching her testimony. Sean Combs’s lawyers say they may need to call Fine as a witness later. The prosecution pushed back, saying Ventura needs her support system while on the stand. They met in the middle: The prosecution agreed that Fine would leave for Ventura’s testimony regarding the allegation that Combs raped her in 2018.

Daniel Phillip is done with his testimony. It is likely that Casandra Ventura is up next.

The prosecutor questioning Daniel Phillip, Maurene Comey, acknowledges that Casandra Ventura may have appeared to enjoy herself during their sexual encounters. But she doesn’t want the jury to forget the testimony about Sean Combs’s role, and his violence.

“Through all of your sexual activity with Cassie, who was in the room?” Comey asks.

“Sean Combs, myself and Cassie,” Phillip replies.

“And is Sean Combs the same man who threw a liquor bottle across the room when Cassie asked him to wait a minute?” Comey responds. Phillip says yes.

The prosecution’s second witness, a man named Daniel Phillip who said he was paid to have sex with Casandra Ventura in front of Sean Combs, is finishing his testimony on the second day of the trial. The defense tried to elicit testimony from him showing that Ventura was a willing participant in these encounters. After Phillip finishes on the stand, we are likely to hear from the prosecution’s star witness: Ventura herself.

The cross-examination of Daniel Phillip has ended, and the prosecution is getting another chance to question him.

The cross-examination of Daniel Phillip, who was paid to have sexual encounters with Casandra Ventura, is getting into the nature of his relationship with the singer. It is certainly a complicated one.

Phillip said he could be paid from around $700 to $6,000 for each sexual encounter, which Combs watched. But he testified that he had never been paid for sex before their first night together, and that he “didn’t care if I got paid one way or another.” Phillip agrees that he wanted their relationship to be deeper than just these voyeuristic sex nights.

“Had she ever given me the chance to date her,” Phillip said of Ventura, “I absolutely would have.”

Daniel Phillip is being questioned about how often he saw Casandra Ventura under the influence of drugs. Phillip testified yesterday that it was only once, and at that time, Combs cancelled their planned sexual encounter. Xavier Donaldson, Combs’s lawyer, is trying to highlight that drugs were not a significant part of their sexual relationship. That issue speaks to consent and whether or not there was coercion in the relationship between Combs and Ventura.

Xavier Donaldson, Sean Combs’s lawyer, just finished questioning Daniel Phillip, who said he was paid to have sexual encounters with Casandra Ventura, on what the witness has said was the first time he saw Combs abuse her.

Phillip testified yesterday that Combs threw a liquor bottle that missed Ventura and then dragged her by her hair into a bedroom. The defense is lingering on an important part of Phillip’s testimony: that after the abuse, Combs said, “Are y’all ready to continue,” referring to the sex. Donaldson is suggesting that Phillip told law enforcement something different in an interview in late 2023: that Combs told him to leave the hotel after the dispute.

The reason this is so important is because prosecutors are trying to prove that Combs forced or coerced Ventura into sex. If Combs pushed Ventura to continue with sex after he beat her, that could be powerful evidence.

In his questioning, Sean Combs’s lawyer Xavier Donaldson is trying to establish that Casandra Ventura was a completely willing participant in sexual encounters with him.

Was Ventura the one who paid you? Yes, Phillip replies. She wasn’t drunk or high? Phillip agrees she was not.

“Is it fair to say she was in complete control of everything she did?” Donaldson asks.

“I cannot say that,” Phillip says.

Daniel Phillip, who said he was paid to have sex with Casandra Ventura while Sean Combs watched and masturbated, is back on the stand. He is being cross-examined by one of Combs’s lawyers, Xavier R. Donaldson.

Casandra Ventura, the singer and model known as Cassie who was Sean Combs’s on-and-off girlfriend, is expected to recount for a federal jury on Tuesday how Mr. Combs instituted a system of abuse and control over her life and career for more than a decade.

Prosecutors say Mr. Combs dangled ever-disappearing music opportunities; beat her when she stepped out of line; and plied her with drugs, forcing Ms. Ventura to have marathon sex sessions with male prostitutes while he taped the encounters.

Lawyers for Mr. Combs, 55, who has pleaded not guilty to charges of sex trafficking and racketeering conspiracy, have portrayed the relationship as loving but deeply toxic while maintaining that any sexual arrangements were completely consensual. They depict Ms. Ventura, 38, as a bitter ex and extortionist who sought only a payday, not justice.

What both sides cannot disagree about is that it was Ms. Ventura’s decision in late 2023 to pour her allegations into a federal lawsuit, which ended with an eight-figure settlement, that led to this moment, in which Mr. Combs has fallen from a beloved billionaire celebrity to an inmate facing a potential life sentence.

When she takes the stand on Tuesday, prosecutors will hope to portray Ms. Ventura as an imperfect but brave truth-teller. She will be asked to explain the context surrounding some of the most explosive pieces of potential evidence — including her would-be memoir and graphic videos of sexual activity and abuse that prosecutors will argue are linked by the web of Mr. Combs’s power and dominance.

Surveillance footage from 2016 that jurors first saw on Monday shows what prosecutors have said is Ms. Ventura attempting to escape a hotel room, only to be brutally beaten and dragged by Mr. Combs in the hallway. She is also expected to testify that, near the end of their relationship in 2018, Mr. Combs forced his way into her home and raped her.

Ms. Ventura was a piano player and dancer as a child, becoming the first student body president at an arts school in Connecticut and composing an original opera, “The Two Faces of Friendship.” She then pursued modeling, walking the runway at New York Fashion Week and appearing in catalogs for Abercrombie & Fitch, J.C. Penney and Delia’s.

After graduating from the Williams School, a Connecticut prep academy where she had scored a dance scholarship, in 2004, Ms. Ventura moved to New York to follow her show business dreams. She met Mr. Combs the next year, and in 2006, signed a 10-album deal with his record label and released her debut single, “Me & U.”

Despite intermittent songs with stars like Lil Wayne and Akon, plus a 2013 mixtape released online, Ms. Ventura never put out a proper second album.