The official described how assassination attempts on Kim Jong-il, the country's leader until three years ago, drove the country's feared internal security apparatus to take elaborate measures against suspected plots, real or imagined - including an attempt by a rogue army unit to launch missiles against Pyongyang.
The official, who asked to be named only as "Mr K", said he had personal knowledge of two assassination attempts on Kim Jong-il, who ruled North Korea from 1994 until his death in December 2011.
In one attempt, a lone gunman with an automatic weapon attempted to shoot him, but was captured before firing. In another, a would-be assassin driving a 20-ton lorry rammed his motorcade but failed to kill Kim Jong-il, whose car was in a convoy of identical limousines and was not among those damaged.
In an extraordinarily rare behind-closed-doors briefing, the official also detailed two attempted coups against the regime, following uprisings in the Korean People's Army, especially among the officers who had been trained in the former Soviet Union.
He described how North Korean officers who had graduated from Moscow's Frunze military academy had been persuaded by Russian officials to feed intelligence back to the Kremlin.
In one plot, a group of officers hoping to provoke a Russian intervention against the regime planned to stage a bomb attack on the Russian consulate in the North Korean city of Chongjin. In another, a north eastern army unit planned a missile strike on key targets in Pyongyang. Both plots, said Mr K, were discovered before they took place.
Mr K's claims cannot be directly verified, but much of what he said is supported by other sources. He requested that the directorate that he worked for and his current activities in South Korea remain secret.
Both accounts appear to be supported by circumstantial evidence. North Korea watchers have noted that in 1994, a group of officers who had studied in Russia were rounded up and imprisoned, in what became known as the "Frunze Affair".
Then in 1997, for reasons that were at the time unexplained, the regime sent troops into the headquarters of the army's Sixth Corps, prompting firefights and arrests. The corps was subsequently disbanded.
Describing the country's internal security system, Mr K - who fled the country in 2005 - said that even the most senior cadres and army generals were routinely monitored, often by agents posing as their chauffeurs, and their activities reported to the Supreme Leader in weekly bulletins.
He also said the regime's crackdown on private markets had led to a flurry of dissident graffiti and pamphlets, with messages such as "How are we supposed to survive?" scrawled on walls.
So suspicious were Pyongyang officials that when a circus in the North Korean capital burned down a day before Kim Jong-il's birthday, it was believed to be an anti-regime protest, he said.
The country's notorious gulags, meanwhile, are run by a unit with the chilling cover name of "Farm Guidance Directorate" - appropriately enough, said Mr K, because the prison-camp inmates are "less than human".
In the "total control" camps, where political criminals are sent, "once you check in, you do not check out," he said. "Even dead bodies do not leave the total control zones."
The two assassination attempts on shortly before he took over as leader from his father, Kim Il-sung, help to explain his subsequent paranoia, and his preference for traveling by private, armored, train, the intelligence official said.
He described an incident in which Kim Jong-il was talking with senior officials inside a ruling-party compound when there was an electricity blackout.
Instantly, bodyguards tackled all the officials to the ground and surrounded the leader.
Whether the plots were real, or imagined by a paranoid regime, is unclear. "I would be sceptical unless you have a chain of collaborative evidence, and in a state which applies torture, you can create collaborative evidence by skillful application of the hot iron," said Andrei Lankov, a North Korean expert at Seoul's Kookmin University. "But this does not mean conspiracies did not exist."
Unlike most defectors, who remain slight because of years of malnutrition and who are often quiet and nervous, Mr K has a robust build and appears to be in his late 40s. He spoke confidently, using expansive body language.
He had little information about the country's current leader, Kim Jong-un, he said, but speculated that he would be difficult to assassinate. "Anyone meeting the supreme leader is patted down, everyone," he said. "I would guess even family members."
During his public appearances, the younger Kim is protected by a triple cordon of bodyguards, state security agents and regular police. The locations he visits are carefully vetted in advance.
Mr K said that while North Korean agents are present among South Korea's 25,000-strong community of defectors, he does not fear assassination.
"I do not raise my voice against the regime," he said, though he admits he always keeps an eye out when taking public transport, and is wary if he sees another vehicle following his in traffic.
Relaxing at a late-night Seoul food stall after his briefing, Mr K admitted that he misses some things about Pyongyang: friends, colleagues and Korea's native grain spirit, soju.
North Korean soju, he opined, is far stronger than its South Korean counterpart - which, he growled, tastes "like water".