The press in South Korea is full of feverish allegations of cult rituals and the mysterious influence of an old friend of President Park Geun-hye. The BBC's Stephen Evans in Seoul looks at the personalities behind the scandal, and whether it could end up toppling a president.
Only the two women involved in this unprecedented political storm know the true dynamic of their relationship. And President Park and her confidante Choi Soon-sil are not telling.
Choi is suspected of exploiting her links with Ms Park for her own financial gain and has now returned to the country to face her accusers.
In the absence of information, the rumour mill is whirling. Newspapers and television channels have been feeding the frenzy of speculation, throwing out ever more fanciful conjecture about a mysterious relationship that goes back four decades.
Is the mentor really a malign influence reminiscent of Rasputin - Ms Choi's father was certainly labelled the Korean Rasputin by some - or is she just an old friend?
A lonely job
Both Ms Choi and Ms Park are intriguing characters.
The current president is the daughter of Park Chung-hee, a military strongman who took power in a coup but who then decided that his brutal dictatorship should become a modern, industrialised democracy. Warts and all, he is the father of modern South Korea.
He was assassinated by his own head of intelligence in 1979. The motive remains unclear but one theory is that the director of the Korean CIA, who fired the fatal shot, felt he had fallen under the influence of Choi Tae-min, a pseudo-pastor.
Choi was also the father of Choi Soon-sil, the woman at the centre of the current controversy.
Five years before the president's assassination, the current president's mother had also been killed, shot dead in central Seoul by a sympathiser of North Korea.
The daughter immediately returned from her studies in Europe and became, in effect, first lady, accompanying her father on his official duties.
With both parents the victims of political violence, the daughter then went into politics on her own account, where she too was physically attacked. She had her face slashed so badly at one political meeting that it needed 60 stitches. The long, thin scar is still visible, running from her right ear to her lower jaw.
On top of this extraordinary background, Ms Park, who is not married, has no close family to lean on in what, in any case, would be a tough and lonely job in the presidential palace, known as the Blue House.
Is this why she leans so much on Choi Soon-sil?
The mysterious mentor
Ms Choi's father, Choi Tae-min, was a colourful character, married many times - six, according to South Korean media - and a frequent changer of his own name.
He was a Buddhist who converted to Christianity and then set up what he called the "Church of Eternal Life". In this guise he became a friend of the Park family, both of the senior President Park and of his daughter, now president, as she was growing up. Mr Choi's daughter became a friend of the young Park Geun-hye.
The two young women remained friends and that is the relationship now under scrutiny.
According to the New York Times, the founder of the religious sect won his way into favour with the now-President Park by saying her assassinated mother had appeared in his dreams, begging him to help the orphaned daughter.
But this is not the first time this unusual relationship has spilled into public view.
An American diplomatic cable from 2007, leaked by Wikileaks, said: "Park has also been forced to explain her own past, including her relationship some 35 years ago with a pastor, Choi Tae-min, whom her opponents characterize as a 'Korean Rasputin', and how he controlled Park during her time in the Blue House when she was first lady after her mother's assassination".
It is this allegation of a mystical influence on the current president which is doing the political damage.
Prosecutors are investigating whether funds set up by Choi Soon-sil are within the law - the unproven allegation is that the funds sought money from companies by putting pressure on them to pay because of her presidential connections.
That may or may not be true, but corruption has rarely destroyed political lives in South Korea in the past.
What may be politically fatal this time is the suggestion that sensitive documents were passed by the president to the mentor for approval.
A South Korean cable TV station says it got hold of computer hard drives from a vacated office previously used by Ms Choi and they showed that President Park submitted speeches to her friend for alteration and approval.
President Park has already apologised, though for what isn't quite clear. Her head bowed, she said: "Regardless of what the reason may be, I am sorry that the scandal has caused national concern and I humbly apologise to the people."
"Choi advised me on expressions in my speeches and public relations during the last presidential campaign and she continued to help me for a certain period of time after I took office," she admitted.
The advice was only before she had a formal system of advisors in place after moving into the Blue House, she insisted.
But the apology hasn't satisfied President Park's opponents or even some in her own party and it is very likely that the anger has not yet subsided.