The Lazarus Pit was a natural phenomenon of mythical vats of arcane chemicals which had blended together to create a revolutionary regenerative serum that was used most commonly used by Ra's al Ghul to ensure his longevity and eternal youth as well heal all of his wounds instantaneously.
The substance that filled each pit was a chemical blend of unknown composition, that seemingly originated from somewhere within the Earth's crust. It bubbled to the surface only at certain key points around the globe, typically at the junction of key lines.
Although capable of healing any injury, restoring youth and even restoring life, the pits had the temporary side-effect of casting the dweller into temporary aggressive insanity. This effect did wear off after a short time.
The Lazurus Pit was used by Ra's al Ghul for over 600 years after he learned how to create it himself after discovering what chemicals it needed to renew and restore life. Over the years, Batman tried to find out what Ra's was using to keep himself alive and giving him great power, but he never found out until the events of Arkham City.
Between Arkham Asylum and Arkham City Incident
Arkham City Incident
During the chaos created by the Joker and the other villains during the events of Arkham Asylum, the League of Assassins were able to recover Ra's al Ghul's body from Dr. Young's Office. They returned to Wonder City, where they dumped the body into the Lazarus Pit, and revived their master to a decayed and weakened state.
Needing Ra's' blood as a component for the cure to the fatal Titan poisoning that Joker had infected him with, Batman faked wanting to join the League of Assassins. Upon hearing Batman's adamant refusal to take his life and therefore gain control of the League of Assassins, Ra's used the Pit again, and regained his prime physique. After defeating Ra's, Batman warned him against continuing the use of the Pit as it was distorting his mind from the centuries of use.
Some time during Batman's battle with Clayface, Joker detonated the Monarch Theatre floor, and caused Batman and Clayface to drop into the Lazarus Pit Room in Wonder City. They both continued to fight, but Clayface remained at the back of the room. While Batman did battle with some clay monsters, Clayface would also try to attack Batman by spitting clay. But after several shots from the Freeze Blasts, he was frozen solid again. Batman went in to grab the vial with the cure, and cut through Clayface. Seeing no way to get the cure from Batman, Joker prepared to take a dip into the Lazarus Pit. However, Batman intentionally threw Talia al Ghul's Sword onto the platform that the Joker stood on. By doing this, the platform fell into Clayface and into the Pit itself, which destroyed it, killed Clayface, and prevented the pit from being used again or from getting into the wrong hands, such as the Joker's.
After Arkham City Incident
After the main game was finished, if Batman went back to the spot where Ra's body was found, his body was missing. Similarly, Talia al Ghul's body mysteriously vanished after the fight with Clayface. An explanation for this was that the League of Assassins may have attempted to revive them using the Pit, but were unsuccessful as it was destroyed.
Before Arkham Knight Incident
The League was successful into resurrecting their leader as they planned to attack Gotham City. However, they used an impure version of Lazarus. This left their master in a zombified state with minimal brain activity and an inability to form complete sentences. The only way he could live is if he would be constantly receive an influx of Lazarus chemicals. The League set up a life support system under Elliot Memorial Hospital. Due to his unhealthy state, several assassins suggested killing Ra's and letting someone else take the title of Demon's Head. This caused a rift in the League. The Loyalists sought to fully revive their master, while the Rebels wanted to ensure that the Demon's Head was never brought back.
While investigating a police report about two dead ninjas, Batman was tasked by the Loyalists with finding a pure source of Lazarus chemicals to heal Ra's or else the war between the assassins would destroy Gotham City.
Batman used the Batmobile to track the source of Lazarus under Panessa Studios. After finding the last pool of pure Lazarus on Earth, he was attacked by the Rebels. After defeating them, he took a sample of Lazarus and attempted to destroy the rest with Explosive Gel. He was then stopped by the leader of the Rebels, Nyssa Raatko, Ra's al Ghul's second daughter who took his detonator from him. She wanted to take over the league to ensure that no more innocents would be hurt and she would force her assassins to leave the city forever. Nyssa then detonated the explosive gel, destroying the Pit. Nyssa then asked Batman to destroy her father's life support and let her take over.
If Batman decides to give Ra's the Lazarus cure, he injects him with the last Lazarus on Earth. Then, the Rebels attack intent of killing Batman for betraying them. After Batman defeats all of the Rebels, he is attacked by Nyssa asking why he would help a man as evil as Ra's. The Pit grants Ra's his strength back. In a fit of rage, he slashes Nyssa's stomach. After a brief battle with the Dark Knight, Ra's escapes, now with his intelligence and stamina returned. Batman asked Nyssa if she wanted to try using the impure Lazarus to heal her. She refused noting that she would just end up like her father. Since there was now no more Lazarus, Ra's was left to live his final life.
If Batman decides to destroy Ra's life support, he uses two Batarangs to destroy the Lazarus machine. Then, the Loyalists attack intent on killing Batman for his condemning their master to die. After Batman defeats the Loyalists, Ra's tries to crawl away. He is then confronted by Nyssa, who is disgusted by what he has become. She attempts to kill her father, but she is stopped by Batman. She tells the Dark Knight that her father will die without the Pit. She says that he has a few days at most. Batman takes Ra's to the Gotham City Police Department. After locking the former Master of Assassins away, Ra's thanks the "Detective" (Batman) for finally saving him from his addiction to the Pit.
- Healing: Small doses of the formula heals most injuries and ailments in a second. When Batman was at death's door, the chalice of the Lazarus formula restored his health and healed almost all of his infection, thereby giving him several hours of extra life.
- Reanimation: Ras al Ghul has resurrected may times from even the most serious of deaths.
- Insanity: Upon the first few minutes, the user of the Lazarus becomes insane and starts to hallucinate deeply.
- Physical Augmentation: The Lazarus can heal natural defects of the human body and bring all their physical factors to their peaks.
- Mystical Empowerment: The Lazarus formula may have some mystical properties, as Ras Al Ghul's tattoos glowed due to its effects.
- As Ra's used lightning bolts to hit the Lazarus Pit. It was possible that he did this as the night that Cyrus Gold was reborn as Solomon Grundy in Slaughter Swamp, a storm began, so he most likely used electricity to enhance the chemicals of the pit. This was also backed up by an Arkham City Story entry that revealed that he had experimented on Grundy.
- According to the Arkham Files, the Lazarus Pit was once Slaughter Swamp and the birthplace of Solomon Grundy.
- It's possible there was once a small amount of Lazarus under the Lady of Gotham isle, as this would explain how Deacon Blackfire was able to live so long.
- It was possible that it was the blood of an ancient demon, as Ra's implied in his boss battle by saying a biblical quote that concerned demons: "We are Legion!"
- The idea that the Lazarus pits are connected to a demon was used in the film Justice League Vs Teen Titans, where the pits are revealed to be the creation of Trigon, Raven's demonic father; Ra's al Ghul became his minion after passing away.