In exchange for sparing her life, Sandra agreed to bear David's child and leave them in his care for him to train.

[1] David trained Cassandra in combat, but never taught her to read, write, or speak, rendering her illiterate and mute for most of her life.

In David's first appearance in No Man's Land, Two-Face hires him to assassinate James Gordon.

In One Year Later, Robin captures Cain and brings him to the League of Assassins as ransom to save Cassandra, only to learn that she has become their leader under the influence of Deathstroke's mind-altering serum.

[1] In the 2008 Batgirl mini-series, Cain is revealed to have survived and begun working with Deathstroke to "cripple the meta-hero community".