A 21-year-old inmate was killed at Pentonville prison in a bloody battle to control the wing's "lucrative" contraband route, a court heard.
Jamal Mahmoud was allegedly murdered by three other prisoners in a dispute over a package of phones, SIM cards and a knife.
He had a "key cell" on the top floor that gave him power over illicit weapons, drugs and mobile phones that were delivered by drone, jurors were told.
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.
He was killed with calculated brutality by a group of men armed with weapons that went to find him and those with him
Mark Heywood QC
The alleged killers - Basana Kimbembi, 35, Joshua Ratner, 27, and 31-year-old Robert Butler - 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 holds 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 second most shocking aspect of this death is that it took place in one of Her Majesty's prisons, on a wing populated with people who had been there for some time, overseen 24 hours a day by regular staff.
"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, it was said. By mid-October last year, a "serious dispute" had developed over this access, the court heard.
One of the defendants later told 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".
Mr Heywood said: "Whatever he meant by those words, the fact is he did. He did so within about 24 hours."
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, to tell the victim: "I was gonna come up and see you this afternoon." Mr Mahmoud allegedly replied: "It has gone past talking now."
Jurors were shown CCTV and given a virtual tour of the prison, although the prosecutor said they would get the opportunity to see it "in reality" later on.
The defendants have denied murder as well as wounding a second man, Mohammed 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. The trial continues.