A man has been jailed after killing his 77-year-old grandmother at her Buxworth home.
Dorothy Bowyer was stabbed to death at the house on Western Lane alongside the family's dog Captain, in the early hours of February 14.
Her grandson William Blunsdon was jailed at Derby Crown Court on Monday for ten years and four months, after admitting manslaughter on the grounds of diminished responsibility.
He was also ordered to serve an extended five-year licence period on his release, after Judge Nirmal Shant QC assessed him as a dangerous offender.
Judge Shant told Blunsdon: "Psychiatrists agree the most likely diagnosis is one of perisistent delusional disorder and the defence of diminished responsibility reduces culpability from murder to manslaughter.
"At the time of the act you were acting on the belief your grandmother and the dog were imposters and your grandmother intended you and your grandfather harm."
In what the judge described as evidence of premeditation, Blunsdon armed himself with a bayonet-style knife the night before the assault.
Blunsdon stabbed his maternal grandmother five times in the chest and once “through the mouth” in her bedroom, the court heard.
He also shot the family’s former mountain rescue dog Captain - known as “Cap” - with a crossbow, before repeatedly stabbing the animal, also in the delusional belief it was an imposter.
Michael Auty QC, for the prosecution, said: “He plainly formed the intention to kill both the dog and his grandmother. There was an element of planning and premeditation.
“He plainly knew what he was doing at the time, albeit in the erroneous belief that his true grandmother had been replaced by an impostor, who was also a witch.”
Consultant psychiatrist Dr Manjit Gahir concluded the 26-year-old was suffering a “persistent delusional disorder”.
Jaime Harrison QC, in mitigation, said Blunsdon was “open and willing” to treatment and medication for the disorder.
Detective Chief Inspector Sally Blaiklock, of the East Midlands Major Crime team, said: “This incident has had a devastating impact on the family and our thoughts are with them at this time.
“Although no sentence can ever ease the tragic loss felt by this family, we thank them for their patience and understanding during this investigation. They have endured accounts of the terrible events that have led to the death of Dorothy and their dog Cap, and have conducted themselves with absolute dignity throughout.”