Superman was a hero of Earth, until Barbatos transformed him into a gigantic, hideous monster, and forced him to aid in the destruction of the multiverse as the mindless Kryptothrax, The True Doomsday.
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Other Personas of Clark Kent (Dark Nights: Metal)
History
Superman and the Justice League stood against Barbatos' invasion of the multiverse. However, when they attempted to don armor made of Element X to battle him, they found it corrupted. The armor enslaved the heroes to Barbatos, and transformed them into horrible beasts, with Superman becoming a large, rotund monster named "Kryptothrax".
For two years, these Dragons of the Bat waged war on what was now called "Earth-Metal", until they encountered the returning Duke Thomas, now called the "Last Monitor". Kryptothrax attacked Thomas, but he escaped with the aid of Nightwing. The Last Monitor and Nightwing rallied the remaining heroes of Earth-Metal to stand against the Dragons, with Kryptothrax falling to Nightwing's Kryptonite guitar pick. In his last moments, the monster remembered who he once was, and thanked the hero for destroying him.