A self-confessed devil worshipper has been locked up indefinitely for stabbing his 18-year-old cousin to death.
Mosa Abid, 20, wrote a rambling letter to Satan pledging to kill his only friend Moamen Settar before carrying out the attack on October 15 last year.
He went to Mr Settar's home in Wembley north London and armed himself with a large knife from the kitchen before repeatedly stabbing him, the Old Bailey heard.
The victim's two younger sisters, aged nine and 15, heard screams and shouts coming from their brother's bedroom before Abid left the house laughing with the knife hidden behind his back.
The sisters found their brother lying on the floor of his room with four or five stab wounds to the chest.
Meanwhile, Abid went to his father's flat in Golders Green with a cut on his hand and changed into a new white shirt.
He told one of his father's neighbours that "The Agency" had tried to kill him, the court heard.
The next morning, police found him sitting on a bench by a church in Northolt with a knife up his sleeve.
Police searched his home in nearby Dehavilland Close and uncovered documents detailing his interest in devil worship and the Illuminati, including the letter to Satan stating his murderous intent.
Prosecutor Oliver Glasgow QC said the defendant had a number of mental health problems and was diagnosed with an autism spectrum disorder in 2010.
He was convicted in October 2015 of using violence to enter premises and battery.
In June of that year, he had broken into a neighbour's home and assaulted a 15-year-old girl claiming he was working for the Illuminati and accusing her of taking photographs of him.
He received a sentence of 16 weeks in a young offenders' institution, suspended for 12 months.
The court heard that Abid and his cousin, who was of Iraqi descent, were close even though there had been incidents of violence in the past.
Two years before the killing, Mr Settar's mother had found Abid with his hands around her son's neck in an apparent attempt to throttle him, the court heard.
Abid, who was diagnosed with paranoid schizophrenia as well as autism, pleaded guilty to the manslaughter by reason of diminished responsibility and was sentenced to a hospital order without limit of time.
In a victim impact statement, father Abbas Kadum said the family was still "not okay" six months after the killing of his only son.
It had led to a breakdown in relations between the two sides of the family and left his daughters "very scared" that Abid would come back and kill them when he is released.
Sentencing, Judge Philip Katz told the defendant: "It is clear you had to have some very distorted thinking and a very, very unhealthy interest in Satanism.
"Your interest in Satan and the writings found in your room show you had already thought about killing your cousin.
"The crime you committed was brought about by your mental state, in the context of which, you wrote you were going to kill your cousin in a long and rambling letter to Satan."