Esteban Corazon de Ablo was a powerful alchemist in the 9th Century who sold his soul to the demon Mephisto to lengthen his life far beyond a human span. Years later, Diablo set up a base in Transylvania, but the villagers there turned against him and buried him within a stone crypt under his castle. Another century later, the Fantastic Four appeared in Transylvania on a vacation. A local, Baron Hugo, told them the legend of Diablo. That night, Diablo whispered to the Thing to awake and free him from his prison by pulling out the stone plug on his crypt and smashing the stone slab that covered him. Freed, Diablo offered a potion to the Thing that would turn him halfway into a human, and in exchange for Ben’s services for a year, he would give him the other potion that would finalize the change.
When the rest of the Fantastic Four arrived, they found Ben working for Diablo. They fought with him and tried to convince him to leave his service, but Ben refused, and the four were forced to leave him behind. Diablo soon made his appearance known to the world, and sold his alchemic chemicals internationally, making him the richest man on the planet. Shortly before the rest of the world, Mister Fantastic discovered that Diablo’s chemicals worked, but only for a short while before failing. The world turned on Diablo, but he had already built up an army of loyal followers and was prepared to use his alchemic powers to conquer the planet.
But once Diablo had revealed his true intentions, the Fantastic Four were free to attack him. At the same time, the Thing had also discovered that the potion he was given wore off, and he turned on Diablo, but Diablo used one of his chemical pellets to knock him out and seal him in an unbreakable, glass capsule. The remaining three members of the Fantastic Four soon attacked Diablo’s fortress, but after a struggle, Diablo managed to capture them with his chemical pellets. Diablo sealed up the last of the Fantastic Four in the same capsules, but Ben quickly escaped when his broke down. Angered, he chased Diablo through his own castle until he crushed a suit of armor into a giant ball and threw it after the alchemist. Diablo ran from it, hiding within the crypt that the villagers had sealed him in. The Thing followed behind him and reinserted the giant, stone plug, and then smashed it with both fists, causing the entire castle to come crumbling down upon the crypt. The Fantastic Four were later freed, and the Human Torch melted the stone into slag to harden Diablo’s prison.
But Diablo eventually used his potions to destroy his melted, stone prison, and escape to revenge himself upon the Fantastic Four. His next move was to travel to North America and New York's State University, where he helped Professor Gilbert to animate his android creation, Dragon Man. He turned the Dragon Man against the Fantastic Four, but it eventually turned on him and drove both of them underneath a frozen lake, where he was lost for a time. Diablo was also later responsible for the destruction of their penultimate headquarters, Pier 4.