WARNING: The following contains spoilers from "Legacy," a story in Detective Comics #1027 by Tom King, Walter Simonson, Laura Martin, and John Workman, on sale now.
Being Batman is an incredibly dangerous commitment, since Bruce Wayne comes perilously close to death on an almost nightly basis. But after all the villains who almost brought him down, one minor villain appears to have finally done the deed, even if it takes a couple of decades to kick in.
Now, Detective Comics #1027 reveals that Doctor Phosphorus is ultimately responsible for Batman's eventual death by exposing him to massive amounts of radiation during a fight, which leads Batman to eventually develop a terminal case of cancer.
Created in Detective Comics #469 by Steve Englehart and Walt Simonson, Doctor Alexander Sartorius was a member of a board of scientists who sought to build a nuclear power plant within Gotham. After being exposed to a massive amount of radiation, Sartorius became a walking radioactive reactor, with his body glowing and his skeleton appearing through his skin. After threatening to poison the Gotham Water Supply as revenge for his fate, Doctor Phosphorus became an enemy of Batman and has fought multiple Gotham heroes in the Post-Crisis DC Universe. He would later appear in the Post-Flashpoint DC Universe, briefly serving as the king of the underground city Charneltown.
But in Detective Comics #1027, Doctor Phosphorus reveals that the radiation has finally caught up to him and left him with lethal amounts of cancer. There's no hope for Doctor Phosphorus, he decides he won't be going alone. Doctor Phosphorus challenges Batman to a one-on-one fight, under threat of finally unleashing his full power and leaving Gotham a radioactive crater. Batman arrives and follows the additional rules Phosphorus has set for the confrontation: Batman can't wear any special armor or use any unique technology. He has to face Doctor Phosphorus man-on-man in a fistfight.
Although Phosphorus doesn't stand a chance against Batman in a straight fight, he's able to keep the brawl going for a surprisingly long time. As he reveals to the Dark Knight, this was by design. Rather than defeating Batman with his own two hands, Doctor Phosphorus has instead decided to drag on the fight as long as possible to force an exposed Batman to continue being in direct contact with highly radioactive material. By doing so, Doctor Phosphorus ensures that Batman's cells are bombarded with enough radiation to give him cancer. With his actions, Phosphorus has all but guaranteed that Batman will develop an incurable form of cancer. If nothing kills him before then, this will be the final fate of Batman.
Phosphorus reveals he's done this specifically so Batman will remember him on his death bed -- not Joker or Riddler or some other notable villain -- but the oft-forgotten Doctor Phosphorus. As Batman defeats Phosphorus, he reveals that the Wayne Foundation has been making progress with cancer patients, and he promises to get him enrolled in the program while he's in Arkham. Even though Doctor Phosphorus may have very well killed him, Batman still decides to help, because that's his legacy. Tragically, the story does confirm Doctor Phosphorus' plan to cement his legacy by terminally wounding Batman worked.
Past glimpses into the future reveal that Bruce Wayne -- now far older and living with his wife Selina Kyle -- has been afflicted with cancer, now revealed to be the what Phosphorus promised would come for him. This version of the DC future has been hinted at repeatedly over the years, especially across Tom King-written storylines. In this future, Bruce Wayne does indeed live a long and eventually happy life before he develops the disease, watching the Bat-Family grow and expand and become their own people. He and Selina have a daughter, Helena, who carries on her father's cause. And eventually, Bruce passes away, leaving the older Catwoman as a widow.
Now, it's confirmed that Bruce Wayne's cancer came as a result of his heroic career. The scene is finally shown, with Bruce laying in bed and holding Selina's hand as he passes away, Doctor Phosphorus's legacy finally coming to fruition. It's a bittersweet moment for Batman and Catwoman, one final human moment that finally takes one of the most vital heroes of the DC Universe. But Bruce isn't shown in pain, but rather smiling at his wife as he fades into the Great Beyond. It's a fitting and quiet end for Bruce and his mission, and probably one of the happiest possible conclusions for the character to pass away peacefully.