SEOUL—Botox injections, a secret list of blacklisted artists, and a horse named Vladimir: The 90-day investigation of South Korea’s political scandal seemed to air the nation’s dirty laundry.

In their corruption probe of Samsung executives, President Park Geun-hye and dozens of others, the special prosecutor’s team seized over 900 mobile devices, computers and hard-disk drives, and pored through 16 terabytes of data—roughly equivalent to more than 60 maximum-storage iPhones. The investigation benefited from a suspect turning over a tablet allegedly owned by one of the key figures, a friend of President Park.

The manufacturer: Samsung Electronics Co.

Updates from the prosecutor’s office amounted to must-watch TV for many South Koreans, with each new revelation causing the disbelief typically reserved for a Korean drama. The daily press briefings made a celebrity out of spokesman Lee Kyu-chul, a bespectacled former judge, whose taste for colorful scarves and long winter coats prompted endless sartorial debate.

The lead prosecutor, Park Young-soo, earned a nickname with local media: the chaebol grim reaper. Chaebol refers to a large, family-run South Korean conglomerate; of these, Samsung is the biggest.

In a 99-page report Monday, prosecutors shared some of what they had learned. The 122-person team conducted 46 raids, questioned Samsung heir apparent Lee Jae-yong four times and ultimately indicted 30 people.

They alleged that President Park and her friend, Choi Soon-sil, engaged in phone conversations using devices registered under different names. In total, prosecutors said the two women spoke by phone 573 times over a roughly six-month period last year—averaging more than three calls a day.

Ms. Park and Ms. Choi have denied wrongdoing; on Monday, Ms. Park’s lawyer dismissed the prosecutor’s report, calling it “fiction.” Samsung has acknowledged it made payments to entities allegedly linked to Ms. Choi but said they weren’t in exchange for political favors. Mr. Lee has been indicted on bribery and other charges; he too has denied wrongdoing and his trial begins Thursday.

A doctor close to Ms. Choi, Kim Young-jae, said he gave President Park at least five Botox injections from 2014 to 2016 at the presidential Blue House, without leaving official records, according to prosecutors. President Park has denied she received any Botox treatment.

During the investigation, prosecutors said, they learned of a secret list—compiled under orders by President Park’s office—of 9,473 writers, actors and singers who had been critical of the Park administration or the president’s father, Park Chung-hee, South Korea’s former military dictator.

Vladimir was one of the horses allegedly purchased with Samsung funds, intended for the training of Ms. Choi’s equestrian daughter, according to prosecutors. Ms. Choi’s daughter, Chung Yoo-ra, won gold at the 2014 Asian Games and aspired to compete in the 2020 Olympics. Ms. Chung was detained in December by authorities in Denmark, where she remains. South Korean prosecutors have an arrest warrant for Ms. Chung valid through 2023. Through her lawyer, Ms. Chung previously has denied wrongdoing.

It is unclear what has become of Vladimir.