A middle-aged man was attacked and needed 30 stitches after he complained to neighbours about a rowdy party.
The 54-year-old went to ask millennium revellers in a flat in Iceni Way, Shrub End, Colchester, to quieten down when he was hit with what is believed to be a bottle.
He was taken to Colchester General Hospital with serious cuts across his forehead and into his right eyelid.
He received stitches and was still recovering on Tuesday from the terrifying ordeal.
Police said it was the most serious incident they had to deal with in Colchester on New Year's Eve.
They were called to the flat in Craig Court just before 10pm.
Two teenagers were arrested on suspicion of causing grievous bodily harm but later bailed pending further inquiries.
Detectives would like to hear from any witnesses who have not yet contacted them.
Det Sgt Roger Napier, of Colchester Police, said: "It was a nasty attack.
It appears there was a dispute about a noisy party and the incident happened after a man went to complain about the fracas."
Anyone with information can call Colchester Police on 01206 762212 or Crimestoppers anonymously on 0800 555111.
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