Testimony in the Sean “Diddy” Combs sex trafficking trial has opened a window into what prosecutors say was the sordid world of group sex, drugs and violence beneath his glittering, jet-set persona.

The government’s star witness wept, telling jurors that the rap mogul’s “freak offs” made her feel “worthless, just like dirt.”

Casandra Ventura, the singer known professionally as Cassie, ended four days of sometimes grueling testimony about being abused by Mr. Combs.