Dionesium is a rare and unique naturally occuring chemical-substance that can be found buried deep beneath the Earth in the Prime DC Universe. Considered by many to be the key to true immortality, it is capable of healing fatal wounds and injuries and can even be used to revive the dead should the worst happen.
It originates from the Dark Multiverse.
At an uncertain point in history, the insane Demon God Barbatos created Dionesium in the Dark Multiverse and sent it off to Earth as part of his future plans.
The first known occurence of a creature coming into exposure with Dionesium was a neanderthal that would come to be known as the super-villain Vandal Savage. Vandal came into contact with a meteor enfused with Dionesium and as a result he became immortal and gained superpowers.
Thousands of years after Vandal Savage found it, Ra's al Ghūl came upon a large amount of impure Dionesium which he tampered with and named The Lazarus Pit. Ra's whould go on to found the League of Assassins and used the Pit to survive for hundreds of years past his natural lifespan. This corrupted form of the substance however also corrupted Ra's al Ghūl's mental-state with every use of it and drove him to madness. Ra's would also start finding and constructing more and more Lazarus Pits around the world for his personal use.
At some point in time, Dionesium was obtained by the secret Gotham society the Court of Owls. The Court invested large amounts of money to start mining underneith Gotham to find a Dionesium spring but the only vein which they could find was corrupted and impure. They used the Dionesium to create the Electrum which they used to ressurect their assasins the Talons and give them their superpowers.
In the 21st century, Dionesium was officially researched by three scientists working for Wayne Enterprise reffered to as "The Doctors Three", those being Prof. Hugo Strange, Dr. Karl Helfern and Dr. Paul Dekker. Helfern and Strange both suspected that the substance came from meteors but Dekker thought otherwise and eventually managed to officially discover the material. Dekker named the substance Dionesium after the Greek God Dionysus who was associated as the god of rebirth and who himself was ressurected. Paul would later on take up the villain alias of "Crazy Quilt" and create a Dionesium medical-thread which he called "The Healing Stitch" that he used to illegally experiment on people, often with disasterous consequences. Crazy Quilt used this to get Batman's attention as he believed Batman was an immortal only to be defeated and realize Batman was mortal.
Years later, a nearly-dead Joker happened upon a large lake of pure Dionesium in a cave system deep beneath Gotham and exposed himself to it in such a way that his body began naturally producing the substance from his spine. Joker then hunted down Dr. Dekker and persuaded him that he was an immortal like Vandal Savage or Ra's al Ghūl in order to persuade the man into creating a Joker Virus with the Dionesium so that there would be no normal cure. After having started his Endgame, Joker lied to Dekker and told him he'd make him immortal as well with a tainted form of Dionesium which Crazy Quilt willingly injected himself with only to die.
The pure Dionesium lake was eventually hunted down by Batman who used the chemical to cure the Joker Virus and after a bloody fight with the Joker, he seemingly destroyed it by collapsing the cave. Both Batman and Joker were killed in the collapse but the small amount of Dionesium which the rocks absorbed made both men come back to life, albeit without their memories. This amnesia did not last forever however and they both regained their old identities.
Dionesium was one of the five metals that, when all touched to a single person, could create a portal to the Dark Multiverse. Unknown to Batman, he was cast to be the portal and was purposefully exposed to Dionesium in his fight with the Joker.
- Although never stated, it is very likely that there was Dionesium in the Slaughter Swamp which ressurected Cyrus Gold and brought him back as the superhuman Solomon Grundy. Crazy Quilt, while describing the substance in Part IV of Endgame, even says some stories account for Dionesium being found "some special substances in the soil, vegetation or water", possibly directly aknowleging Solomon Grundy.
- Crazy Quilt states that the legends of Macrobia, the River Styx and the Water's of Gilgamesh were all different cultures accounting for occurences of Dionesium.