N.R.G. Kim Hwan Sung, Known to fans as an "obsidian idol" for his dark skin, died suddenly in 2000 at 19, days after he had complained of a sore throat.

His death, which remained shrouded in mystery, shocked fans throughout Asia and became one of the first major tragedies in the short history of modern K-pop.

Kim made his debut and joined NRG, which stands for New Radiancy Group, in 1997.

At 16, he was the youngest in the group and became known for his delicate features, his long hair — a unique look for a male idol then — and charismatic stage presence.

With members Lee Sung Jin, Chun Myung Hoon, Noh Yoo Min, and Moon Sung Hoon, Kim made acrobatics an art form in K-pop performances of NRG that generations would follow.

The group rose to fame swiftly, selling over 200,000 units with their debut release and winning the New Face Award at the 1998 SBS Popular Song Awards.

Their second album, Race, elevated them to international status, especially in China and Japan.

At the time, few Korean acts were promoted overseas, making NRG one of the trailblazers of the first Korean Wave, or Hallyu.

But their meteoric rise came to a tragic halt in 2000.

Shortly after promotions for the group's third album, Kim told fellow member Noh Yoo Min that he had a sore throat.

His condition quickly worsened. He experienced severe abdominal pain and a high fever, and doctors were initially unable to identify the cause.

Kim was eventually diagnosed with malignant viral pneumonia and fell into a coma. He was declared brain-dead days later.

On June 15, 2000, his parents made the difficult decision to remove him from life support. Kim died that day, just two months shy of his 20th birthday.

The precise cause of death remains undetermined. Some fans and media have speculated that Kim may have contracted an early, undetected form of SARS while promoting in China.

However, the first known SARS outbreak wasn't officially documented until 2003. Because his body was cremated the day after his death, an autopsy that could confirm or deny these theories was never performed.

In the years following Kim's death, NRG went through multiple lineup changes and hiatuses. Still, the remaining members continued to honor his legacy.

During their 2001 concert, the group performed two tribute songs, "Good Bye My Friend" and "Antonio"—the latter a reference to Kim's Catholic name. In 2002, NRG won their first-ever music show award with "Hit Song," fulfilling one of Kim's dreams.

In 2018, all four surviving members reunited on the talk show Video Star, marking their first appearance together in 13 years.

They spoke candidly about Kim's influence on their lives and the weight of his absence.

"He was our youngest, but also our heart," Chun Myung Hoon said during the reunion.

Lee Sung Jin, who left the group in 1999 and later faced controversy over gambling charges, has since reconnected with fans and even shared plans to start a business.

Noh Yoo Min now runs a coffee brand, NoUMin Coffee, and occasionally appears on TV.

Moon Sung Hoon resurfaced in 2012 after a long hiatus and now runs a fashion brand.

Today, NRG's surviving members continue to perform and share stories of their late friend, ensuring that Kim Hwan Sung's memory remains an enduring part of K-pop's history.