Diablo was born in Spain in the 9th century. He sold his soul to the demon Mephisto for a longer lifespan than a normal human. He then went to Transylvania and set up a secret base there but he soon was captured by the villages and buried him in a stone crypt.
Mayor Story Arcs
Versus the Fantastic Four
The Fantastic Four had a holiday in Transylvania. A man named Baron Hugo told them about Diablo and the ancient cockpit which buried him. Diablo communicated with the Thing and offered him a deal. The deal was if the Thing would liberate Diablo from his imprisonment, he would give him the cure of his ugly form and to make the Thing human again. The Thing agreed and went to the stone crypt and freed Diablo. Diablo proposed a new deal with the Thing saying that he would give him the potion in exchange for the Thing's service. The Thing accepted the deal.
The Fantastic Four discovered that Ben had been working for Diablo and tried to force Ben from his service with Diablo. Ben refused to leave with the Fantastic Four which caused the other three members of the Fantastic Four to desert Ben.
Diablo eventually sold his chemicals to the world. His chemicals were effective and useful which made him famous and wealthy. He then began to gather a group of loyal followers and empowered them with his chemicals. Mr Fantastic studied the chemicals Diablo made and concluded that Diablo's chemicals only worked for a short period of time. Diablo, realizing that his secret had been exposed to the public, used his loyal followers and attempted to conquer the world.
At that time, the potion Diablo gave to Ben was beginning to wear off. Once again back to the abomination Ben never wanted to be. The Thing, angered that he had been fooled by Diablo, wanted revenge on Diablo. He broke into Diablo's castle and tried to kill him but he failed and Diablo imprisoned the Thing in an unbreakable glass prison cell. The Fantastic Four also went into the Diablo's fortress and tried to stop Diablo but instead were also imprisoned in the same type of prison cells which Diablo used against the Thing. The Thing broke free from the cell and chased Diablo. Diablo was very afraid and tried to run away from the Thing. The Thing squashed a steel suit into a steel ball and hurled the ball towards Diablo. Diablo escaped from the metal ball and hid inside the crypt in which he had been imprisoned. The Fantastic Four trapped him in the crypt once again and strengthened the crypt.
Diablo's Second Return
Diablo used his chemicals and broke free from the crypt. He wanted to kill the Fantastic Four for imprisoning him again. He met a scientist called Dr. Gregson Gilbert and both of them worked together as partners. Both of them succeeded in animating the Dragon Man and Diablo tried to use it against the Fantastic Four. The Dragon Man betrayed Diablo and both of them were taken to the depth of a frozen lake and was then lost in time.
Diablo was recently contacted by the Future Foundation, alongside other older Fantastic Four villains, to try to find a way to defeat Reed Richards.
A Council Of Masters
Max Fury, the leader of the Shadow Council has assembled a new Masters of Evil in the sovereign nation of Bagalia where Diablo is presently a member. Diablo is present when Max Fury and the Masters of Evil capture John Steele after he attempts to escape Bagalia with the Serpent Crown and the Crown of Thorns.
Earth-1610 (Ultimate Universe)
In the Ultimate Universe Diablo's real name is Menendez Flores and also goes by the aliases Dr. Diablo, El Diablo and Maestro. He was born in 15th century Genoa, Italy and taught himself alchemy, herb lore and 15th century science. He first appeared in Ultimate Fantastic Four #39 (2007).
Years later, Diablo has lost a lot of his powers and has aged a great deal. He recently tried to use the Fantastic Five to develop a new type of elixir that would restore his youth and powers properly but was defeated by the team.
Earth-691 - Guardians of the Galaxy
In this alternate reality Diablo has managed to stay alive in the 31st century but he has seemingly lost his mind. His alchemy still seems to be working and he uses a potion to heal Charlie-27 who has been incarcerated in the same mental asylum in which he lives.
Earth-295 - Age of Apocalypse
Diablo and Crusher Creel (The Absorbing Man) are back up guards for Apocalypse's Infinites (Genetically-Engineered soldiers) at the ruins of a Mayan temple in Central America, that had been converted into a prisoner of war camp. Magneto had killed the Infinites in his attempt to rescue Robert Kelly, who was being held prisoner there, causing Diablo and Crusher Creel to attack him. Magneto is aware of who they are and mentions to himself that Diablo is "said to be centuries old and has mastered the forgotten art of Alchemy," through which he was attempting to trap Magnus with the ground under his feet. Diablo controls the vines around them and grabs Kelly, while Crusher knocks Magneto flying. Crusher and Magnus brawl, meanwhile Diablo creates a double pointed dagger in his own hand after Kelly tries to talk his way out of futher fighting, still tied up by the vines. As Diablo is about to plunge the dagger into Kelly's chest, Nightcrawler (Kurt Wagner) teleports in, surprising Diablo and blocking the dagger with a bladed gauntlet covering his forearm. As this happens, Diablo calls Kurt an elf, to which Kurt replies "this from a man wearing antlers on his head!" and kick to the face defeating him.