It happened on the evening of Thursday 31 October 2019 when a 15-year-old boy sustained serious cuts to his head.
It is believed some of the guests had travelled to the party from the Scarborough and York area.
Officers are appealing to anyone who was present at the party, held at a house on Walton Close, to come forward if they have not already spoken to the police.
Anyone with any information is asked to contact North Yorkshire Police on 101, select option 2 and ask for Northallerton CID. Or email firstname.lastname@example.org
If you prefer to remain anonymous, you can call Crimestoppers on 0800 555 111.
Two 17-year-old boys were arrested in connection with the incident and have been released while under investigation.
The victim received hospital treatment for a number of cuts to his head.