David Cain, one of the world's premier assassins, was by the nature of his profession a very lonely man, and began thinking about what he would leave behind when he died. He wished for a "perfect child" - specifically a "perfect artisan of his craft".
Cain was at this time a member of Ra's al Ghul's League of Assassins. Ra's had created the League to be "The fang that protects his head", but he had grown tired of the uncertain loyalties of its members. When he learned of the theories Cain was developing to train a perfect killer, Ra's was intrigued, and supplied Cain with infant test subjects in the hopes of creating a new generation of assassins. This experiment proved disastrous; those infants that survived eventually turned on one another until only one boy remained. Ra's ordered one of his "Ubu" guards to kill the boy "like a mad dog". The guard, however, took pity on the abused child, and secretly set him free in the woods instead. Cain and Ra's al Ghul did not know that the boy still lived, and Cain began a new experiment, raising his own daughter, Cassandra, to replace his earlier failures.
Ra's' daughter, Nyssa Raatko, did know that "The Mad Dog" had survived. Years later, after Ra's' death, she sought out The Mad Dog and made him a member of her new League of Assassins. She took him on as a protegé, telling him everything about the experiments, and about who he was intended to be.
Cassandra gathered evidence indicating that Lady Shiva was her mother, and sought out Shiva to confirm this. At the time Shiva was the sensei of Nyssa's new League. When Batgirl arrived she played a key role in the rebirth of Mr. Freeze's wife Nora Fries as the monstrous Lazara, and several League members died in the resulting chaos. Due to the conflict between their loyalty to Shiva and Nyssa and their near-worship of Batgirl as "The One Who is All" the League split at that point, with several members pledging themselves to Cassandra. Several more members of the League (including all the defectors except one) died when The Mad Dog went on a killing spree. The Mad Dog was successful in killing Batgirl (who gave her life to protect the last of the defected assassins), although she was able to knock him unconscious before perishing. Cassandra was quickly restored to life in a Lazarus Pit by Shiva. The Mad Dog's fate is unknown.