Super Friends: Rest in Peace

Batman is dead, killed with a crystal called Noxium, recovered by one time Legion of Doom member Dr. Natas and buried under the Hall of Justice by Superman in Kryptonian steel, a harmless form of Kyrptonite. Shown in a flashback when Superman inadvertently opens the casing after Luthor gets it, the Legion of Doom plans to kill the Super Friends as Batman was in the same way as the crystal holds each of the individual weaknesses that can do all of the Super Friends in.