Sordid sex marathons featuring gallons of baby oil. Physical abuse so savage that the victim was left bleeding and vomiting. A threat to blow up a romantic rival’s car.

Casandra Ventura’s testimony against Sean Combs, the music mogul who was her longtime boyfriend, during the first week of his criminal trial in Manhattan federal court was a depiction of untrammeled decadence. It spared neither the defendant nor the witness herself.

Ms. Ventura’s account of a life defined by Mr. Combs’s desires came in the early stage of what is expected to be an eight-week trial. Her testimony was a first step toward convincing the jury that Mr. Combs was not merely an abusive lover, but the leader of a criminal enterprise that carried out the sex trafficking of three women and committed arson, kidnapping and other crimes dating to 2004.

Whether a jury sees Mr. Combs as merely a violent voyeur or a criminal kingpin depends on more than shock value.

“You can be guilty of sins and not crimes,” said Donna Rotunno, a defense lawyer who represented Harvey Weinstein, the disgraced Hollywood producer, at his first sex-abuse trial.

In a trial that will feature more witness testimony and reams of other evidence, potentially including videos of Mr. Combs’s sex parties, the government will now build on the foundation provided by its star witness.