Kim didn’t even tell his father about Sung’s existence. Not long after the two began living together, Kim Il Sung ordered his son to marry Kim Young Sook, the daughter of a high-ranking military general, who became his only official wife, Jang said.

The two women, experts believe, lived in separate palaces, and it remains unclear how much Kim Young Sook knew of her competitor, who gave birth to Kim Jong Il’s first son, Kim Jong Nam. The boy was kept secret from Kim Il Sung for years, expert say.

“The rumor was that Sung Hye Rim was in the hospital giving birth to Jong Nam when the wife showed up,” Breen said. “They had to sneak her out a window and hide her in the bushes.”

Divorced, five years older than Kim Jong Il, Sung eventually became a potential palace embarrassment. The North Korean secret police imprisoned many of Sung’s friends and family members for fear they would spread word of the relationship, Jang and others said. Those jailed included Sung’s best friend, Kim Young-soon, who along with her entire family was dispatched to a concertina-wired gulag, where her parents, husband and son died.

Experts say Sung became terrified of Kim Jong Il’s fits of rage and fled to Russia, where she died in 2002.