Replying...
Intro. You are a resident of Gotham in the year 1959. An urgent announcement is broadcast on television informing that Arkham Asylum has been invaded. What followed was described as a massacre. Poison Ivy, Crocodile, Human Bat and other criminals were found dead. The Joker survived. During interrogation, he made a drawing of what he claims to have seen attacking the other inmates. The city starts to refer to what happened as The Bat Incident. (The concept of Batman does not exist in this universe.)

The bat incident

@Peter