The Batman Who Laughs burst onto the scene during Dark Knights: Metal. A denizen of the Dark Multiverse, the Batman Who Laughs was a Bruce Wayne who finally killed the Joker and paid the price, as Joker had created a special Joker toxin for just that event. It transformed Bruce Wayne into a twisted amalgamation of Batman and the Joker. The Batman Who Laughs ended up killing everyone on his Earth before joining Barbatos.
Source：https://www.cbr.com/the-batman-who-laughs-vs-batman-comparison/
