The Death Knight is a new character for Batman I wanted to create. He's not human but an android created by the Joker who resembles Batman. Since Batman was dubbed the Dark Knight, the Joker calls his prize creation the Death Knight.
The Joker wanted to use the Death Knight to destroy Batman. However, the robot believed himself to be so superior that he imprisoned the Joker and cloned his creator to use as a servant. While the real Joker was trapped inside a dungeon, the clone crippled Barbara Gordon, murdered the second Robin, and murdered Sarah Essen-Gordon. The Death Knight ordered the clone to commit those crimes and saw himself as the real Batman.