The Joker is rendered sane in The Lazarus Pit (Batman: Legends of The Dark Knight #145)
After being shot multiple times by Talia Al Ghul, Batman has to make the difficult decision to save The Jokers life. The Dark Knight has to save his life in order to find where Ra’s Al Ghul and Talia have gone….
The Lazarus Pit has restorative capabilites, such has bringing the nearly dead back to life and restoring youth, vigor and vitality to the aged. However to some, after emerging from The Pit, undergo a brief period of insanity.