DC Database

Bruce Wayne, AKA Batman, created a potion to cure the dual personality of Harvey Dent.

When a massmurderer called the Joker comes up, he tries to find out who it is. He later finds out it's him when he takes the potion.

As the Joker, Selina Kyle and Harvey Dent were in danger of being killed. The Joker shoots Selina leaving her paraplegic, before doing the same to Harvey Batman tries took take control of his body and kill "The Joker" personality.

By doing so, he dies as well, by dying he passes on the Batman mantle to Harvey.