"All of creation cares not whether you live or die."
The Key to the Well of Souls is a plot item featured in Darksiders II.
The key opens the door to the Well of Souls in the Tree of Life and Death. In order to protect it, as the power of the well could not be entrusted to any one power, the key was split in two and each half given to the care of Heaven and Hell. The Angel Key was entrusted to Archon Lucien in Lostlight while the Demon Key came into Samael's possession in Shadow's Edge.
After discovering the Crowfather's spirit in the City of the Dead, Death journeyed to obtain this key so that he could access the Well of Souls and restore humanity to the balance. He eventually recovered the Angel Key after Archon Lucien, having been driven mad and fallen under the influence of the Corruption, slaughtered most of the angels in Lostlight and attempted to kill Death with the Rod of Arafel.
To find the Demon Key, Death had to travel to Samael's fallen fortress and then into the past. After an intense duel, the Blood Prince gave Death the key and remarked that the horseman's upcoming confrontation with Absalom would be interesting. When Death brought both keys to the Crowfather, the old one directed him to the grand doors set into the Tree. The horseman used Soul Splitter and inserted the keys, opening the doors and allowing him to proceed to the well and his final encounter with Absalom.
The current location of the key is after Death's sacrifice is unknown.