|“|| The Well of Souls is a place of unimaginable power... the key to life and death, of creation itself. |
— Crowfather, explaining the well.
The Well of Souls is a supernatural dimension that is guarded by the Archangel Azrael. It is said to hold power over life and death, and it acts as a receptacle for the souls of the departed. Once a soul is sent to the Well, it remains there until passing through the City of the Dead, where it is essentially recycled in preparation for rebirth. After it is cleansed in the City, the soul then re-enters the Well and remains there until the time for it to be reborn comes to pass.
After the premature Apocalypse had been initiated, The Destroyer, posing as his former persona of Abaddon, manipulated Azrael and the Archon Lucien into using the Angel Key to partially open the Well. Abaddon then betrayed Heaven by using the Well to empower his forces, and imprisoned Azrael in his newly-constructed Spire. With the power of the partially accessed Well, Abaddon routed the Hellguard and secured his place as ruler of Earth.
Perhaps because of Abaddon's tampering with the well, the soul of the Nephilim Absalom was able to contaminate the well, and released his anger and desire for destruction against creation in the form of Corruption into the Abyss. Eons later, during his quest to save his brother War, Death was told by an otherworldly merchant called Ostegoth to seek access to the Well in order to resurrect the Human race.
To gain this access, he traveled to the Dead Kingdom and called upon the services of the Lord of Bones. After completing a set of tasks for the Lord of Bones, Death was granted passage to the City of the Dead, where he found and killed a being known as the Wailing Host. Though Death initially believed that this deed would enable the souls of Humanity to be reborn, he was told by the spirit of Crowfather that a dark power is drawing energy from the Well (the Destroyer), and that he would have to physically enter the Well and provide a sacrifice of billions of souls to repair it.
The Crowfather then informed Death that, in order to enter the Well, he would have to retrieve the Angel Key and the Demon Key from their respective keepers. The Angel Key, in the hands of the now-Corrupted Archon Lucien in Lostlight, was retrieved by Death after slaying the mad Archangel. Death then traveled to Shadow's Edge, Samael's seat of power, to retrieve the Demon Key.
Upon his arrival, he was told by the Demon Queen Lilith that Samael had vanished, and with him the Key. Giving Death the Phasewalker, she instructed him to use this power to visit Shadow's Edge before Samael's disappearance, and retrieve the Key from him. Death managed to locate Samael in short order, and, upon meeting him, engaged in a short skirmish with him. After very slightly wounding Samael, Death was given the Key by the intrigued Demon, who then departed.
Death then proceeded to the Tree of Death and confronted Absalom inside the Well. Defeating Absalom for the final time, Death then chose to sacrifice himself and the Souls of the Nephilim in order to return man to the Earth. Unfortunately though Death's task was only half done, though repaired the fact of the matter was that the Destroyer still had unbridled access to the Well of Souls and used them to fuel his army.
Decades later, War was summoned by the Charred Council on the pretext of being punished for his supposed role in initiating the End War. Vowing to the Council that he would hunt down and destroy the real instigators of the disaster, War eventually made his way to the Destroyer's Spire, where he meet the imprisoned Azrael. Aiding Azrael's escape, War then killed the most powerful of the Destroyer's lieutenants, Straga. In doing so, he also destroyed the Spire itself, cutting off the Destroyer from the Well's power and allowing War to confront and kill him thus completing Death's mission.