The Joker- Batman #38 "Endgame" pt. 4
During a visit to Arkham Asylum, Batman is locked in The Joker's old cell by an orderly named Eric Border. Border reveals himself to be The Joker in disguise before paralyzing Batman with knockout gas. When Batman recovers from his paralysis he discovers that The Joker has released an airborne toxin through Gotham that mutates its victims- making them look identical to The Joker. Further, the drug replaces feelings of love with feelings of hate and violence which causes chaos.
Batman heads to Gotham Presbyterian Hospital while James Gordon does some research. Batman finds a drugged Joe Chill and a re-creation of the night his parents died- revealing that The Joker knows his true identity. James Gordon finds photographs taken years earlier that contain someone that looks strikingly similar to The Joker...
The Joker attacks and Gordon shoots him, presumably killing him. When Gordon calls Batman to recount his triumph, The Joker arises and attacks Gordon with an axe. The Joker takes the phone and says "Hello Bruce". The revelation of The Joker's regenerative abilities is jaw dropping and, while Batman tries to understand his adversary's new dynamic, The Joker breaks into the Bat Cave. The Joker cuts off Alfred's hand and steals Batman's trophies- parading them through Gotham for the amusement of the infected citizens.
I won't ruin the rest of the story arc because I've already said enough. The important thing about Batman #38 is the realization that The Joker is immortal, granted eternal life by radiation from the same meteor that gives Vandal Savage and Ra's Al Ghul their longevity.