A 21-year-old inmate was stabbed to death at Pentonville prison in a bloody battle to control the wing's "lucrative" contraband route, a court heard.
Jamal Mahmoud was allegedly "executed" by three other prisoners in a dispute over a package of phones, SIM cards and a knife.
The new father, part of a group known as The Somalis, had allegedly warned "if you want war, I will give you war," after being threatened with a large combat knife.
The alleged killers - Basana Kimbembi, 35, Joshua Ratner, 27, and Robert Butler, 31 - were allowed out of their cells, even though prison authorities had been warned of looming trouble.
Opening the Old Bailey trial, prosecutor Mark Heywood QC said the attack on October 18 last year shocked the other prisoners at jail which houses category A and B offenders.
He said: "He was killed with calculated brutality by a group of men armed with weapons that went to find him and those with him.
"That lethal violence was quite deliberate and expected. When it happened, both sides knew that it was going to happen and were prepared to engage in it. In the event, the level of it was shocking, shocking to those who lived in that place.
"The sheer determination of the killers is indicated by the fact that neither the location, the security or the presence of prison staff served to stop it or deter the violence or the weapons used to drive it home."
He said the defendants killed Mr Mahmoud to "make their point" and get "control of part of the lucrative contraband route onto that wing of the prison".
Jurors were told how the defendants and victim, nicknamed Chaos, were housed in G Wing at the north London jail, which houses more than 1,000 prisoners.
Mr Mahmoud occupied a key cell on the top floor which gave him power over the influx of illicit phones, weapons and drugs as they were delivered by drone or other methods. By mid-October last year, a "serious dispute" had developed over this access, the court heard.
One of the defendants was to later tell prison authorities that Mr Mahmoud was "playing with the big boys", jurors were told. The day before the killing, a package had come into the prison and there was open talk of a "beef", jurors heard.
A civilian employee saw a length of sheet dropped from the fifth floor window over the prison wall into the street before it was pulled back, jurors heard.
Later, the victim was seen by another inmate to visit Butler and Kimbembi in a cell, jurors heard. During their discussion, Kimbembi allegedly pulled a large combat knife from a sheath and told him to move back.
Mr Mahmoud left, saying "we'll see what happens" and Kimbembi allegedly looked up and said "I am going to kill you". Later, Mr Mahmoud told his wife about the trouble over the parcel in a phone call and told her he was irritated because it was "his operation".
The next day, Mr Mahmoud told her he still felt "violated" and she told him not to do anything stupid, jurors heard. He also told an inmate that he had confronted the men and told them: "If you want war, I will give you war." The inmate allegedly approached a prison officer and warned not to open the defendants' cells or there would be trouble.
Supervising officer Dizzy Vergo spoke to the defendants and told them they may be victims of something but let them out after they gave her assurances, jurors were told.
Shortly before the killing, Kimbembi was overheard by another inmate, Bobby Dorset, telling the victim: "I was gonna come up and see you this afternoon." Mr Mahmoud allegedly replied: "It has gone past talking now."
Just 10 minutes before the murder, Ms Vergo spotted Ratner behaving in an "unusual" way. She called him over and warned she would not tolerate anything happening on the wing. The defendant told her nothing was going on, "lying" to her face, jurors heard
The three defendants, who had access to weapons, made their way up to the fifth floor, where Mr Mahmoud was attacked in an area not covered by CCTV shortly after 3pm. According to Mr Dorset, Butler punched Mr Mahmoud's associate Mohammed Ali in the face then pulled out a flick knife and jabbed towards the victim.
Kimbembi then allegedly ran at the pair with his hunting knife. He stabbed Mr Ali in the back and grabbed Mr Mahmoud before plunging the blade into his chest, according to the witness. Another witness allegedly heard Kimbembi say: "I told you not to f*** with me."
As Mr Mahmoud was being treated, Kimbembi went for him again, pushing prison officers away and kicked him several times, the court heard. Mr Ali had a can in a sock and a kitchen knife was seen to fall from Mr Mahmoud's trousers, jurors heard.
Afterwards, Butler allegedly laughed and said: "We've got him. We've got him. He got what he deserved. That's what happens when you play with the big boys."
Mr Heywood told jurors: "It was effectively an execution."
The defendants have denied murder as well as wounding Mr Ali, with intent to cause him grievous bodily harm. The trial was adjourned until Monday afternoon when jurors will be taken on a site visit to the prison.