Lazarus Pit is one of the most amazing thing DC's Multiverse has. Not only this, it is also a big question now-a-days, especially for those who are shocked by the death of Oliver Queen in the Arrow. So let's see what exactly is Lazarus Pit.
The history is still in progress.
However, the Lazarus Pit is a Natural Phenomenon or a pool filled with Electromagnetic leylines of acids and poisons emerged from the earth's crust. It was discovered by Ra's al Ghul in order to save Sultan's Prince (son). The Pit has the power to heal or say, restore anyone's age. Based on his observation, Ra's al Ghul started to conquer and use the Pit to restore his age. But due to the strong and stressed rejuvenation, it causes temporary insanity or boosts the hatred or vengeance buds. The time spent in Lazarus Pit is proportional to the power and healing level but in addition, the longer one stays in the pit, the dangerous is the stress level.
Characters who used the Pit.
1) Jason Todd (Robin)
Jason Todd or the Robin did not intentionally use it. It was Ra's al Ghul himself who resurrected his dead body into the pit after Joker killed him. Bruce Wayne, on the other hand had no idea of this. Only later he finds out that Robin had become one of the most threat to the Joker known as 'Red Hood'. As featured in the movie Batman : Under the Red Hood, Jason disappears mysteriously by the end of the movie.
2) Black Canary
Black Canary (Dinah Laurel Lance) was once seriously wounded. As shown in Birds of Prey #35, she used Lazarus Pit to regain the Canary Cry and the ability to be a mother again.
3) The Riddler
The Riddler was believed to use the pit to heal his diseases. He used the pit to cure his cancer in the form of brain tumor to which Batman threatened the riddler that he would disclose his use of Lazarus pit to League of Assassins if the Riddler ever revealed Batman's identity.
4) Lady Shiva
Lady Shiva, after fighting with Cassandra Cain, falls into death's arm and is later, resurrected into Lazarus Pit. Cassandra Cain, herself was resurrected in the pit.
5) The Arrow (Oliver Queen)
Oliver Queen or The Arrow, is killed by Ra's al Ghul. He falls into Lazarus Pit and is resurrected to become The Green Arrow.
6) The Joker
Yes. The Arkham's Clown prince's corpse after being killed by Ra's al Ghul, is put in the Pit by none other than our Batman. He becomes even weirder after being resurrected.
There are others such as Kobra, Nora Fries, King Snake, Duela Dent, Wonder Woman, and Bane who returned from death by the lively bath in the Pit. Most of them are defeated by Ra's al Ghul himself. Only Batman and Bane, however managed to defeat him and destroy almost every Lazarus Pit on the earth. Nyssa, his daughter has used the pits from time to time. Talia al Ghul, his second daughter used Lazarus pit for resurrection of Jason Todd.
Ra's al Ghul remains the sole-conqueror of Lazarus Pits.
and everything he does...
It is rumored that Ra's al Ghul will appear in the three DC TV shows The flash, Arrow and Gotham. However, we are waiting for the holidays to get over soon. *sighs*