Arnold Stromwell was a prominent mob boss in Gotham City before turning himself in to the authorities. He also had a nephew, Tony Zucco, who was responsible for the murder of Robin's parents, although Stromwell had nothing to do with that.
History
Arnold Stromwell's criminal ways had gone on since childhood. He was a juvenile delinquent who spent his time on the railroad tracks of Gotham City often accompanied by his younger brother Michael. Though opposed to his brother's actions, Michael nonetheless saved Arnold from an oncoming train that cost him his leg. This event haunted Arnold well into late adulthood ever since.
Stromwell continued on his life of crime, and fulfilled his ambition that he would grow up to become one of the chief mob bosses in Gotham City. At one point in his career, Batman infiltrated his mansion to catch his nephew Tony Zucco, who had since become a primary suspect in the murder of John and Mary Grayson, parents of Dick Grayson. Stromwell truthfully denied being involved as he didn't order his nephew to launch his protection racket as he saw them as not worth it and was disgusted by the murder of the Graysons. After this confrontation when Batman left, Zucco came in and thanked his uncle for not giving him up to Batman. Stromwell scolded his nephew for what he did and disowned him even after some operatives outside the mansion tried to fire on Batman during his departure. Other than this incident and not ratting Zucco out to the police, little is known of Stromwell's criminal career other than that he was involved in illegal drug manufacturing.
Later in Stromwell's life, Rupert Thorne began to pose competition to his criminal empire. The conflict between the two finally erupted when Stromwell's son Joey went missing. Upon confrontation during a parley, Thorne (who truthfully had no relation with Joey's sudden disappearance) attempted to kill Stromwell by means of explosives, but he was saved by Batman. With the help of Michael, now a priest, Batman tried to get Stromwell to reform and turn himself in to the police. Though there were many other factors that influenced his decision, most notably the effect of his manufactured drugs on his own son, he was finally convinced by Michael, who confronted him on the same railroad tracks where the train accident took place. After Batman saved him and his brother from Rupert Thorne as the police arrived, Stromwell started to make a statement to Commissioner Gordon. It is presumed that he surrendered himself to the Gotham Police and lived the rest of his life in prison, but at peace with himself as he was willingly paying his debt to society.