Thirty-five years have passed since two children were found dead within a fortnight in early 1981 in Toa Payoh. Agnes Ng Siew Heok was just nine years old, Ghazali Marzuki was ten. Their killers were arrested the day Ghazali's body was found, though nobody could imagine that day what police investigators were about to discover.

Singapore has had some sensational murder cases but none has come close in terms of the bizarre revelations that emerged through the course of investigations into the so-called "ritual killings" and the trial of Adrian Lim, his wife Catherine Tan Mui Choo and mistress Hoe Kah Hong. Adrian Lim will be hard to beat as Singapore's most cruel, perverse charlatan and heartless killer.

The case shone a light on a surprising reality of Singapore - that you do not have to scratch deep beneath the shiny surface of this clean, modern city state to discover age-old superstitions alive and thriving.

When Adrian Lim decided to pursue his interest in the occult and learn the practices of spirit mediums and traditional witchdoctors called bomohs, he landed in a goldmine that paid off handsomely.

He found no shortage of desperate, naive and gullible people ready to place their faith in a self-styled guru chanting before an altar in his living room and ringing a bell. He called them his "devotees", and they parted with more than their money, jewellery and other valuables when they turned to him for help.

He persuaded numerous teenage girls and young women that they would find everlasting happiness, good health, perpetual beauty and power over the men in their lives if they took off their clothes and had sex with him.

Tan Mui Choo and Hoe Kah Hong were only two of those he tricked. The others included underaged students, bar waitresses and housewives, as well as well-off women complaining of headaches or insomnia, or wanting help to deal with sickness, unhappiness, wavering boyfriends and unfaithful husbands.

Many of Adrian Lim's antics would be laughable if not for his extreme cruelty towards so many of his victims. Under the cover of being in a so-called trance, he acted out his depravity and subjected the women in his life to harsh physical abuse - beating, slapping and kicking them, pulling their hair and hitting their heads against the wall. Many were tortured with primitive and painful electric shock treatments he devised himself. During one such session, Benson Loh Ngak Hua, a young man married to Hoe Kah Hong, was electrocuted.

Adrian Lim's worst crimes of all were the senseless child killings that led to his arrest and that of Tan Mui Choo and Hoe Kah Hong.

The unprecedented mix of murder, perverse sex, the occult and outright trickery proved unique in sparking widespread public interest in this case through the 1980s - from the day the three were arrested in 1981, through their trial and the appeals of the two women, until all three were hanged at Changi Prison on Nov 25, 1988.

Their court appearances drew hundreds of curious people who swarmed the surroundings of the Subordinate Court Complex and the Supreme Court building and waited for hours just for a glimpse of the three murderers.

The Toa Payoh "ritual killings" provided a larger-than-life warning to those too ready to seek supernatural shortcuts to dealing with the unhappy side of life. The story sounds a warning to those who despair over sickness, misery or relationships that sour and are prepared to seek a quick-fix solution from so-called miracle men claiming supernatural powers, performing rituals or dispensing potions.

Adrian Lim was not the first self-styled healer to get into trouble with the law - and he was not the last.

These days, Internet users go online to find help of exactly the sort Adrian Lim offered, and it appears readily available on websites that come complete with testimonials from satisfied customers.

Sadly, some things never change.

• This is an excerpt from Alan John's foreword to the 2016 edition of Unholy Trinity.