The molten ball that is powered by the Well, note the Night Sentinel Statues on either side.
Despite its name, The Well is not a singular place, but rather consists of three arena like structures, requiring demons to be cleared before large double doors open, giving the player access to an Elemental Wraith, strung up on a sacrificial altar, having its energy siphoned into a large molten spherical ball powering the Hell Portal in the center of Argent D'nur. It is uknown if these arenas were originally a part of Argent D'Nur, but upon using The Crucible to free the last Wraith, a demonic head will open up, leading to a drop with a large rune in the middle. Four Night Sentinels apparitions will stand by the entrance, as if guarding it.
Empowered by the energy of the Elemental Wraiths, the Well is a unlimited energy source for Hell, and their presence and usage have made the land of Argent D'Nur revered and heavily guarded in Hell. The UAC processed raw Argent Plasma from this site in their creation of Argent Energy, and had the invasion not occured perhaps the UAC would have relied on the Well for the rest of eternity.
Closing the Well will give the player the opportunity to kill the Spider Mastermind and finish their campaign.
The Well was initially created after a pact formed by the Hell Priest Deag Grav and an unnamed Night Sentinel, only called the Betrayer in the demonic codex entries.
The Betrayer was wrought with grief after the death of his son on the battlefield in the war between Argent D'Nur and Hell, and he was tricked into giving Deag Grav and his Cabal access to the Wraiths in return for having his son back.
The Wraiths were cursed and subjugated, and their subjugation gave Hell the power necessary to best Argent D'Nur, and the realm was absorbed into Hell.
The Well was used as the main power source of Hell until the events of Doom (2016) after which the Doom Slayer used the Crucible to shut them down.
It is unknown what became of the Wraiths or the remains of Argent D'nur after the destruction of the Well.