Detective Chief Inspector Niel Jamieson from West Mercia Police says it is still unclear why Ceri Fuller took the lives of his children.
Recording his verdicts, the coroner said the deaths represented a "great darkness" and were the "sort of thing nightmares are made of".
After offering his condolences to members of the couple's family, the coroner told them: "It's clear to me that the relationship between Ruth and her husband had lasted for about 15 years and was generally quite good.
A discussion about a separation had taken place on July 11, the coroner said, at a time when Mrs Fuller was clearly unwell.
Ruling that it was highly likely that the children were killed on July 12, the coroner added: "What exactly happened on that fateful day may never fully be known.
A two-day inquest heard that the children, from Milkwall, Gloucestershire, were found dead at Pontesbury Hill, Shropshire, on July 16 last year.
All three had suffered a severe neck wound inflicted with the Bowie-type knife, while the two girls had been repeatedly stabbed in the chest.
The inquest heard that Mr Fuller, a paper mill worker who had a degree in molecular biology, drove the children away from his marital home on the morning of July 12 after agreeing to split up with his wife.
During the inquest, Mr Barkley heard that Mr Fuller's wife, Ruth, had been suffering from mental problems and attempted to stab herself and take an overdose after the children left.
Three young children from Gloucestershire were unlawfully killed in a remote quarry by their father who then jumped to his death from a cliff, a coroner has ruled.
The deputy coroner for Mid and Northwest Shropshire described the killing of 12-year-old Samuel Fuller and his sisters Rebecca and Charlotte, aged eight and seven, as acts of "unimaginable horror".
Finding that Ceri Fuller had taken his own life, coroner Andrew Barkley said overwhelming evidence had driven him to the inescapable conclusion that the 35-year-old acted alone in attacking his children with a hunting knife.
An inquest has heard how three children found dead in a disused quarry in Shropshire last summer had each had their throats cut. The body of their father was found nearby at the foot of a 60ft cliff.
Ceri Fuller, who was 35, had taken his children from their home in Gloucestershire to the beauty spot and was reported missing before their bodies were found.
An inquest into the deaths of a father and his three children, who were found dead in Shropshire last July, has started today.
Ceri Fuller, his son Samuel, and his two daughters Rebecca and Charlotte, were discovered at a beauty spot in a disused quarry near Shrewsbury.
Two forensic pathologists announced that all three children had their necks slashed.
Their father, Ceri Fuller, died from injuries likely to be caused from a high fall.
The inquest into the deaths of a father and his three children found dead in a disused Shropshire quarry last summer, will be held over three days in April.
The bodies of Ceri Fuller, aged 35, his son Samuel, aged 12, and daughters Rebecca Rose, aged 8 and Charlotte Mae, aged 7, all from Coleford in Gloucestershire were found in a disused quarry at Poles Coppice, Pontesbury Hill, near Shrewsbury on July 16th.
An inquest into the deaths of a grandmother and her grandson from Rutland whose bodies were found in a lake has found it was a "tragic accident".
Dawn Mullany, 71, from Lincolnshire and Laurence Mullany-Mills, 6, were discovered in Holywell Lake, near Stamford in February.
Post-mortem examinations found they had drowned but despite police inquiries it is still unknown how they came to be in the water though it is thought the pair were near the water to feed ducks.
The six-year-old had gone to stay with his grandmother but had been due to return home earlier that day, the inquest held at Boston Magistrates' Court heard.
The family released a statement saying they knew what the inquest would be able to tell them, now they must rebuild their lives:
Jacqui Thompson's husband Gary was killed when his vehicle was hit by a roadside bomb in Kandahar four years ago. His death was a devastating blow to Jacqui and their five daughters.