Nero is a leader of criminal syndicate called the Frendz Mob, the most powerful criminal organisation in Mega-City One. After surviving but suffering bad injuries after an assassination attempt, his brain has been transplanted into an armoured robotic body and also the Frendz Mob has also been secretly sabotaging the new Judge weapon Mark II Lawgiver Firearms, which the Judges are already being issued with.
After Dredd finds out about the sabotage, before he is able to warn anyone about the sabotage, Nero proceeds with 'Operation Doomsday' which ends up being an all-out attack on the Judges in Mega-City One to overthrow them and take control of the city himself. Although Dredd and Demarco's discovery led to Nero starting his operation early, it was still successful as most of the Judges had been issued with the new Mark II Lawgivers. Legitimate Lawgivers have a safety feature which consists of a sensor in the grip which can detect the palm print of the user and an explosive charge, so that if an unauthorised user attempts to fire one, it will explode in his hand. After Nero transmits a radio signal that will make all of the sabotaged lawgivers explode should the Judges use them. Hundreds of Judges were crippled or killed as a result and Nero proceeds to send the "Assassinator"-class robots city-wide to attack the Judges everywhere. The Judges were in complete disarray.
Later on, the Judges were losing the war badly, with Nero's robots slaughtering Judges and taking Justice buildings. Most of the surviving Judges are in hiding or fighting a guerrilla war since open combat is far too risky. Nero broadcasts the people will be rewarded if they inform on the Judges whereabouts, but promises to execute anyone who aids the Judges. The war is going terribly for the Judges, with Nero's robots being seemingly endless with all the destroyed ones being replaced, the Judges dying in droves, Nero's robots have seized Mega-City One military satellites so he has access to nuclear weaponry and Nero also gained control of the Grand Hall of Justice and the Public Surveillance Unit, enabling him to watch the city and deploy his forces more successfully.
With help from Brit-Cit, Dredd and a squad of American Judges from the embassy go to the Cursed Earth outside of Mega-City One, where Nero has four spaceships land at that location, each containing thousands of Assassinator droids. Dredd and his squad manage to infiltrate on of the ships and reprogram the robots to fight for them and Brit-Cit destroys the other three ships, eliminating Nero's reinforcements.
Without reinforcements and under heavy attack from Dredd's reprogramed assassinators, the Judges fighting back led by Deputy Chief Judge Hershey and Nero's robots suffering heavy casualties, the tide of the war begins to turn for the Judges and Dredd and his squad return to fight in Mega-City One. By the time Nero realises he's defeated, Dredd's robots retake control of Mega-City One's military satellites, denying Nero his doomsday plan. Nero was gunned down in the street by a dozen Judges and they regain control of Mega-City One.