Dwight Junior Freeman, 19 of Tyers Street, SE11 was sentenced at the Old Bailey on Monday, 9 August.
At the same court on Friday, 9 July, he pleaded guilty to the manslaughter of his half-brother, Jahreau Shepherd, on the grounds of diminished responsibility.
He additionally pleaded guilty to wounding with intent in relation to a second male victim.
On the evening of 11 July 2020, Jahreau had been celebrating his upcoming 30th birthday with a party in Venn Park, Kennington.
Around 60 guests were present at the gathering, including Jahreau’s half-brother, Freeman. Not long after 10pm witnesses described seeing Freeman approach Jahreau with two large knives and attack him.
In an attempt to escape, Jahreau ran towards the exit of the park.
He tripped on the stairs leading to Vauxhall Street, where he was stabbed multiple times with both knives by Freeman, who had chased him from the park.
A second man, 26 years old at the time of the incident, was stabbed in the chest by Freeman as he attempted to help Jahreau. He survived his injuries.
Police and London Ambulance Service paramedics attended the scene where Jahreau was sadly pronounced dead.
A murder investigation was launched by detectives from the Specialist Crime Command. Following enquiries, Freeman was identified and officers were able to trace him to Newquay, where he was arrested on 16 July 2020. He was charged two days later with murder, attempted murder and possession of an offensive weapon.
The attempted murder was later reduced to wounding with intent while the possession of an offensive weapon charge is to lay on file.
The court heard evidence from two consultant forensic psychiatrists who concluded that Freeman was suffering from paranoid schizophrenia at the time of the murder, causing a substantial impairment to his ability to understand the nature of his conduct.
Detective Sergeant Ben Dalloway, Specialist Crime North Homicide Team, said: “This is a truly tragic case in which a family have essentially lost two of their loved ones.
“Jahreau was celebrating his 30th birthday listening to music and enjoying a barbeque surrounded by his loved ones. His life was cut short in an extremely violent manner which shocked and appalled those present.
“Jahreau was well loved in the local area and also amongst the mixed martial arts community. He turned to the sport towards the end of his life and was a champion fighter.
“I would like to extend my sincere condolences to Jahreau’s family and friends whilst also thanking the numerous witnesses who came forward to assist the investigation. In particular, I would like to thank those whose quick thinking actions saved the life of the second victim.”
An 18-year-old man was also arrested in connection with the investigation but was subsequently released and no further action was taken against him.