A 17-year-old boy had to have his leg amputated following an "unprovoked" stabbing during a bank holiday trip to Barry Island.
Newport Crown Court heard that the boy, who cannot be named because of his age, was with a group of friends in May 2026 when the incident happened.
Idris Ahmed attacked the teenager after he acted "as a peacemaker" between two groups before the altercation.
The 19-year-old from Riverside, Cardiff denies one charge of grievous bodily harm with intent, one charge of inflicting grievous bodily harm and a charge of possessing a knife.
The jury heard the 17-year-old victim was with friends who were all "having fun, drinking and throwing water and sand at each other" on 25 May, which was a "bright sunny day".
Nuhu Gobir, prosecuting, said Ahmed also went to Barry Island that day with friends and the two groups eventually started to interact close to a rocky area of the beach, with one of the 17-year-old's friends becoming uneasy about the other group.
"The complainant spoke with them and tried to be diplomatic – he was a peacemaker", Gobir said.
The different groups eventually shook hands but the court heard Ahmed returned moments later and stabbed the teenager in the leg "without warning".
"The defendant approached the complainant from behind, pulled a knife from his shorts and stabbed him to his leg without any warning," Gobir said.
He added the 17-year-old victim did not see the person who stabbed him but witnesses did, including those who knew Ahmed.
In the moment after the attack, the 17-year-old fell to the ground and looked pale, saying: "I am going, I feel myself going."
Ahmed then fled the scene and discarded the knife close to a nearby building, an act seen by an eyewitness who later identified the 19-year-old in an identity parade.
Before he threw the object Ahmed was heard saying: "I need to dash this."
He was also captured fleeing the scene by a woman who was taking selfies on the beach with her partner.
The court heard the 17-year-old was taken by air ambulance to the University Hospital of Wales in Cardiff, and had sustained an injury to the main blood vessel in his right leg, with one artery completely blocked.
Despite several transfusions, oxygen was not flowing properly to the leg and a decision was reached that an amputation was necessary.
The court heard the teenager had a through knee amputation – removing the lower half of the leg.
Ahmed was questioned by police shortly after the incident and responded "no comment" to most of the questions, but the court heard he denied stabbing anyone and said he knew nothing about the incident and was released.
Gobir said it was the prosecution's case that the defendant was the person responsible for stabbing the victim and "that he must have intended to cause him really serious harm by stabbing him to a vital part of his body with such a dangerous weapon".
The court was then shown CCTV from various cameras around Barry Island Pleasure Park area showing the groups of teenagers on the day of the incident.
On Tuesday afternoon, the jury were played a police interview with the victim where he explained more about the incident.
He said that some of his female friends had started talking to a large group of boys as they sat on the rocks.
He said that a male friend had started to get nervous, believing he "might have had a problem with these boys" but the teenager tried to calm the situation with the other group.
He said he did not know the reason why he was attacked, describing the sensation of being stabbed as "like a punch", adding "I knew straight away I had been stabbed".
He said it was a "sharp pain" but it "didn't really sting how you'd expect it to".
He described how he was in and out of consciousness as he was taken to the hospital but remembered seeing his friend being attacked by another man with a plastic shovel. He said in total he had to have four surgeries on his leg.
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