Omerta the Silencer has two origins as he was rebooted after the Batman/Huntress: Cry for Blood.
Pre-Cry For Blood
Donald Campbell was the Consiglieri for the Bertinelli family. He was frustrated because Guido Bertinelli never made him a family insider. As Omerta the Silencer he was employed by Stefan Mandragora to kill Guido and his family. He also wrote his name on the wall in the victims' blood. Unknown to him Helena Bertinelli did not die and unknown to her was his identity. After the massacre she got her father's money from Donald himself and when she left he tried to kill her again as Omerta.
Not knowing Donald was Omerta she went into hiding with the help of her old bodyguard Tony Angelo. While hiding he trained Helena who wanted revenge for her families murders. She went back to New York City to find Omerta as the Huntress, and she had a couple run-ins with him, but he was always able to escape.
He eventually tries to fool her by having the newspaper say that her father was actually alive. When Helena gets to the office Omerta shoots at her again, but this time she is able to escape. It is after this event that she realizes Omerta and Donald were one and the same, she then goes back at night to find him. When she tries to get the drop on him she fails and he gets the drop on her instead, captured he plans to rape her but Mandragora stops him. They kidnapped Huntress and take her to a secret safe that belonged to her father. They use Huntress' blood to open the safe and afterwords Mandragora asks her to be his leader in New York City. Of course she refuses to work with someone who killed her father so Madragora shoots and kills Omerta, killing him to prove his serious intention. Huntress still refuses to join Mandragora and she even kills Mandragora herself making Omerta's death completely pointless. A poetic end for the man who murdered at least around a hundred people.
Post-Cry For Blood
He is again the killer ordered by Mandagora, but this time Santo Cassamento is the mastermind wanting to erase Franco and Helena Bertinelli (of which he is the true biological father). This time however, he makes a mistake. Helena's mother was the one intended to be left alive, but he accidentally kept Helena alive instead.