A gang of four led by a 6ft teenager assaulted a man as he walked home early on Thursday.
The four male offenders approached the man, in his 30s, in Beckets Park, Northampton, and demanded money.
Two assaulted, punched and threatened him with a knife before searching his pockets but nothing was stolen.
The victim was taken to Northampton General Hospital to be treated for facial injuries. The main offender is believed to be in his late teens.
He is described as having straight blond hair and wore white trainers, black trousers and a long-sleeved yellow top with black stripes round the chest.
Two offenders described
He also spoke with an Irish accent.
The second offender may have been in his early 20s, he was about 6ft tall with short, black, curly hair.
He wore a light grey tracksuit top zipped up round his chin which had writing on the chest. He was wearing dark trousers and boots.
There are no descriptions of the other two.