Wells of Darkness
The Wells of Darkness are the 73rd layer of the Abyss in the Great Wheel multiverse of Dungeons & Dragons. Having first appeared in the map of the Abyss included in the "Planes of Chaos" boxed set for Planescape, it was expanded upon in Dungeons & Dragons 3rd Edition for the splatbook "Fiendish Codex I: Hordes of the Abyss", it was further developed in the article "Gazing Into the Abyss" in Dragon Magazine #357 and "The Wells of Darkness" in Dungeon Magazine #148, both written by Eric L. Boyd. In the former, rules are presented for using three prisoners of the Wells of Darkness - Ansitif, Astaroth and Cabiri - as Vestiges, whilst in the latter, as part of the Savage Tide adventure path, the players are forced to enter the Wells and release the Succubus Queen Shami-Amoure for advice in their battle against Demogorgon - it also contains rules for Ahazu as a Vestige.
The Wells of Darkness appear as a rocky plain interrupted by jagged tors underneath a steely blue sun that casts dim light. Pocked around the layer are pits seemingly filled with an inky black ichor; these pits are mystical prisons, which bind beings here in a manner that renders them virtually incapable of escape - and certainly impossible to do so without outside assistance. Beings so bound can communicate through a primitive form of telepathy with anyone in close proximity, although the magic rituals used by Binders can also be used to commune with the prisoners here and to tap into their power in the same manner as Vestiges - this suggests that the pits may contain some small fragment of the same mysterious other-realm that other vestiges are known to inhabit.
The Wells are linked by a winding, 10ft wide pathway of marble, and overseen by "The Custodians"; 15ft tall bodaks of exceptional ferocity who never speak, but relentlessly patrol the layer. Atop a 300ft tall tor stands the semiruined stone fortress of Overlook, which allegedly feature a scale model of the entire layer, complete with inky pools marked with the demonic seals of their inhabitants. Many demons believe that the fortress protects some sort of controlling mechanism for the layer, but if so no one has yet figured out how to make it work. Overlook was once the seat of the Demon Prince Ahazu the Seizer, but he grew so obsessed with the binding powers of the Wells of Darkness that he voluntarily fed himself into a vacant well, and nobody has dared take control of the place ever since.
The Wells are a prison for fiends generally considered too useful - or powerful! - to be slain outright, and on rare occasions even non-demons have been forced to make use of the Well's powers to bind creatures or beings here that are too dangerous to allow to go free. Sometimes, as with Shami-Amoure, a creature that could be killed is instead imprisoned because it's far easier to cast true resurrection than free someone from the Wells. Very few beings bound have ever escaped, such as Shaktari, Demon Queen of Mariliths, and Shami-Amoure herself in the Savage Tide adventure path.
Ahazu: Original Demon Prince of the Wells of Darkness, voluntarily bound himself here. It's unclear if he's actually trapped however, as he seems to have an uncanny influence over the black void, referred to in demonologist lore as "Shattered Night".
Ansitif: A Demon Prince of Blasphemy who formed an alliance with six tanar'ri allies to hunt down and destroy an obyrith lord named Malgoth, only for them all to be slain or imprisoned after they had completed their quest.
Apep: A serpentine demigod whose native pantheon believes him to be the ultimate manifestation of evil; they bound him here to prevent the end of existence.
Areex: A forgotten Demon Prince who entered a mysterious pact with a being called Bolothamogg, who is now impatient for Areex to venture "beyond the stars" and pay the price of the deal. Nobody knows who Bolothamogg is or what that deal was. Which is worrying, because gibberlings have begun spontaneously chanting Areex's story and demanding he pay his debt, and the phenomena keeps on spreading... and furthermore, Bolothamogg was explained in Lords of Madness as being one of the Elder Evils!
Cabiri: A many-eyed Demon Prince stated as being imprisoned here by the Queen of Chaos for refusing to join her war against the Wind Dukes of Aaqa in the Fiendish Codex, and described as an obyrith lord that betrayed the Queen of Chaos and sought out the baernoloths only to be imprisoned by them in the Dragon article.
Ebulon: A Demon Prince who once served as a loyal general to Graz'zt before the Dark Prince managed to claim his own layer of Azzagrat; Ebulon ultimately fell in battle against Demogorgon, who dumped his near-dead body in stasis in a Well but then forgot about it. Graz'zt actually does wish to resue Ebulon out of some strange sense of loyalty, but his long delay in doing anything has bred resentment in Ebulon.
Lazbral'thull: A Demon Prince of Torture and Bloodletting who went mad and began preying on other demons; he slew three other fiendish lords, including the demon tree Malgarius, before being humbled and bound here.
Ma Yuan: A colossal, yellow-scaled, four-armed lizardfolk-like creature with the head of a T-rex and a spiked fin, Ma Yuan is aptly known as the Killer of Gods for his power and his deicidal urges. He slew at least ten gods before an alliance of gods and demonic lords (who had also fallen prey to his rampages) managed to seal him away.
Shami-Amourae: Whilst Malcanthet claims the crown of Queen of the Succubi now, that title lay in Shami-Amourae's hands two thousand years ago. She was imprisoned her when her relationship with Demogorgon went sour, as the Prince of Demons realized his lover had learned some of his greatest weaknesses and was using them to manipulate him.
Astaroth: A Demon Prince with the power of prophecy in life, famous for infiltrating the ranks of the Archdevils as "Diabolus" to the extent he became Treasurer of Hell before he was found out and forced to flee, Astaroth was murdered and his identity stolen by an Archdevil named Gargauth, who ultimately went on to ascend to divinity. But, whether because of Astaroth's preemptive attempts to become a god (which, reputedly, Gargauth stole from him) or his pact with Ahazu for his body to be sealed within a Well of Darkness upon his death, Astaroth still clings to existence. Hellfire constantly blankets his pit, incinerating Astaroth's body as fast as it regenerates and keeping him from returning to true life.
Dahak: A malevolent god in the guise of a three-headed dragon, Dahak was imprisoned here by his own pantheon, who had grown angry with his constant attacks on them and their subjects.
Echidna: A goddess who was imprisoned here for her refusal to cease giving birth to terrible monsters, she somehow continues to impregnate herself and bear her nightmarish progeny within the Wells of Darkness. Every so often, Ahazu allows her children to escape their pit and enter the rest of the multiverse.
Kizarvidexus: An obyrith lord who fought alongside the Queen of Chaos, who slew the Wind Duke Soziel only to be slain in turn by another, Icosiol. It is presumably Icosiol's powers that generate the perpetual tornado that rages in the bottom of the pit, sweeping up and scattering the chunks of demonic flesh that ooze from the darkness beneath in order to prevent the obyrith's regeneration and resurrection.
Rr'ikin'aka: A Demon Prince of very little intelligence who dared to challenge Tharzax the Chattering Prince and Obox-Ob the Prince of Vermin (who used to hold Demogorgon's position as Prince of Demons), simultaneously, to try and claim the niche of Demon Prince of Scorpions. Needless to say, they kicked his ass. His pool is buried beneath an endlessly replenished mountain of decaying scorpion corpses.
Ungulex: A tanar'ri who has been imprisoned for so long that none remember who he was or why he was sealed away. Some stories claim he may have been, if not the first prisoner to be sealed in the Wells of Darkness, then one of the first.
Arlgolchier: An ambitious demon who made a pact with Ahazu for power; when he was slain by an alliance of three wizards on the Prime Material, his contract came due and his dead body was transported to one of the Wells of Darkness. Though the Abyss attempts to regenerate him, the pit where he dwells is infested with multiple chasme and abyssal ghouls, who feast constantly on his flesh, preventing it from building sufficient mass and coalescing together. A portal to the Wells of Darkness hidden in the Pit of Prisoners on Pazunia, the 1st layer of the Abyss, emerges here.