The Wall of the Faithless is the large wall that encircles the City of Judgement on the Fugue Plane, made of the souls of the faithless. It plays a major role in the story of Mask of the Betrayer campaign.
The Wall of the Faithless
The Wall was created by Myrkul as a means of disposing faithless souls. Unlike those judged False, this judgement is unwavering; regardless of alignment or deeds, those with no patron deity to claim them were doomed to be absorbed into the wall, facing suffering and eventual oblivion. This suffering is part of the core reasons Akachi and Kaelyn the Dove launched crusades against the injustice of the wall.
When Kelemvor became the God of the Dead, the Wall was one of the few systems of Myrkul's tenure that remained intact. This would weigh heavily on Kaelyn, one of his Doomguides, who could not live with the fact she was giving false comfort to the Faithless.
The Betrayer's Crusade
The first stand against the Wall was launched by Myrkul's high priest, Akachi. Akachi's lover, the Founder, was a wizard who was killed by an accidental spell-backfire. She was a Faithless, and Akachi demanded Myrkul spare her from the wall, which the god refused. In his anger, Akachi rallied many supports, such as his brother Eveshi (who he called Araman), the solar Zoab, the blue dragon Ser'ryu, and the demilich Rammaq. Akachi acquired the Silver Sword of Gith from the Nine Hells and launched a crusade through the Betrayer's Gate in the Death God's Vault, and through that assaulted the Fugue Plane. Akachi managed to tear the Founder free from the wall, and stood defiantly before his god. The crusade however could not stand up to Myrkul's undead horde, and Akachi faced judgement.
Myrkul branded Akachi as False, and as punishment decided to put him in the Wall in the Founder's place. As part of a greater contingency plan, he cursed Akachi with the hunger, and cast him in the Forgotten Realms as the Spirit Eater. This spirit eater would ultimately take on a mortal as a "mask," slowly destroying that person along with many spirits by sending them into the Wall, and would move on to many masks. By becoming a fearful creature of Rashemi folkore, it would ultimately empower Myrkul, by ensuring fear of him would spread; as such, even though he was killed by Mystra during the Time of Troubles, he could still live on as he drifted in the Astral Plane because of the spirit eater.
It was implied by Araman that Myrkul set up the Founder's death to test Akachi, to see who he loved more: his god, or the Founder. Myrkul had gotten his answer.
The Second Crusade
Kaelyn the Dove was one of Kelemvor's doomguides, and could not abide the injustice and suffering of those trapped in the wall. Inspired by tale of the Betrayer's Crusade, she tried to rally forth a new crusade and attempt to bring down the wall and end the suffering of the Faithless.
This crusade did not succeed, and many followers were judged False and were condemned to serve Kelemvor. Kaelyn however was not among those sentenced, possibly due to intervention from her grandfather, but she still persisted in trying to find a new way to crusade against the wall. She left Kelemvor's service and found a new calling in Ilmater's teachings.
The Third Crusade
The Founder had evaded Araman for centuries, having established the Academy of Shapers and Binders in Thay, a Red Wizard academy dedicated to transmutation. After much planning, she arranged for the kalach-cha to be transported to Rashemen and inherit the curse. The key to open the Betrayer's Gate was the Sword of Gith, which kalach-cha used to destroy the King of Shadows. She had the core shard surgically removed from the PC's chest, and fully reforged the blade so he/she could launch a Third Crusade.
During the journey, the PC & Safiya meet up with Kaelyn the Dove in the Death God's Vault, who was trying to to find a way to reach the Betrayer's Gate. She would join up in the third crusade. The original generals of Akachi's crusade, Zoab, Ser'ryu, and Rammaq had reassembled at the Fugue Plane intent on resuming what Akachi started.
The Founder hoped, based on her interrogation of Myrkul's corpse in the Astral Plane, that by having the Spirit Eater retrieve his/her soul from the Wall, Akachi's hunger would be fully sated and he'd be at peace. But Myrkul had lied; Akachi had forgotten himself and became the Faceless Man, an incarnation of hunger. Only by reassembling the Mask of the Betrayer, pieces found in dreamscape from within the fringes of Akachi's memory, could the Faceless Man remember himself and end the curse.
The Future of the Wall
Kelemvor would not allow the PC to destroy the entire wall, insisting it was vital to the order of the universe. He only allowed him/her to retrieve the soul and end Myrkul's curse.
If Kaelyn joined the PC in the finale and wasn't betrayed, then she would be inspired to continue future crusades. Provided Rammaq was destroyed in Eternity's End, she and the Menagerie would continue to lead skirmishes against the wall, freeing some souls from its grasp. Though her wings slowly blacken, she becomes a beacon of hope for the Faithless, who would begin to whisper prayers to her.