Melbourne underworld figure Carl Wiliams has died after being attacked in a prison in Victoria.
His lawyer Rob Stary confirmed Williams' death in the maximum-security Barwon Prison on Monday.
The 39-year-old went into cardiac arrest after suffering head injuries.
Williams was sentenced to a minimum of 35 years in jail in 2007 for the murders of gangland rivals - father and son Lewis and Jason Moran, Mark Amelia and Michael Marshall - and conspiracy to murder another man during Melbourne's underworld war.
The television series Underbelly is based on the gangland killings.