Creation and Inspiration
"The Source Wall as it is currently understood visually in the DCU first appeared in the X/Titans crossover. [...] the Wall as such did not appear in Kirby's New Gods.
What did appear was a four page introductory sequence in NEW GODS #5 in which Metron, using his Mobius Chair, explores the 'Final Barrier'. "And somewhere beyond--lies the Source: the greatest of mysteries." An additional description suggests that this is the Promethian Galaxy--a place of giants--where "all roads to the Source come to an end--". Visually, there's a double page shot of a giant strapped to an enormous piece of Kirby debris. There's another panel with a large head of a female who tried to achieve "the maximum state". And failed. And, in the text, Metron notes that 'beyond all the knowledge and sweeping concept at our command, the mystery of the Source lies--serene---omnipotent--all-wise.
There is, however, no shot of a 'Source Wall' as such. No final visual barrier beyond which an entity may not venture. No wall full of entities who tried to breach the final barrier and failed and became subsumed within the Wall.
Chris and I invented that notion--and its visualization--for the X/Titans book, clearly extrapolating from what Jack had done.
The neatest part of the invention is that apparently, the notion of the Wall and its execution were such a perfect fit for the existing New Gods mythology that folks don't really remember exactly where it came from."
According to Jack Kirby at a San Diego Comic-Con appearance in the late 1980s, the inspiration for the size of the Wall comes from "Rock my soul in the bosom of Abraham", a religious song that contains the lyrics "so high you can't get over it/so low you can't get under it/so wide you can't get around it/you gotta go through the door".
In Countdown, it is revealed the 52 universes are each surrounded by a source wall, with the Bleed between the source walls of the various universes. The Monitors refer to an impending disaster if they don't act to stop travel between the 52 universes, and in issue #38 of Countdown, they finally reveal that each time a being passes between universes, they break a little of each universes' source wall. If a source wall is breached too many times, it will collapse and lead to chaos and the destruction of its universe. In Countdown #23, Superman-Prime operates out of a cave within the Source Wall itself, specifically a section bordering Earth-15. He mentions its deterioration and later blasts it with his heat vision, causing a devastating chain-reaction which seemingly obliterated the 15th universe.
Darkseid's connection to the Source Wall is very notable because he has not only been attached to it, but removed from the wall multiple times. In Jeph Loeb's run of Superman/Batman, Darkseid is thrown on to the wall by Superman and later removed from it because of a deal brokered with an alternate version of the villain. Throughout Darkseid's rule of Apokolips and reign as a new god, he has made it one of his primary goals to breach the Source Wall.
Darkseid's father, Yuga Khan once attempted to unravel the unknowable mystery of the Source as well. However, like so many before him, Khan was made part of the great wall. One of Darkseid's attempts to breach the wall freed Yuga, but his own insatiable hunger for the secrets of the Source returned him to the imprisonment of the Source Wall.
Death of the New Gods
The Death of the New Gods miniseries features The Source Wall extensively, and updates a number of elements in regards to it. In addition to a second Source Wall being created to store the souls of the slain New Gods, the Source itself reveals that it has never truly resided behind the wall, and that only the Bleed rests beyond the wall. The wall is one aspect of the universe that is preventing the Source from merging with its "other half" that was split in to the Anti-Life entity after an attack by the Old Gods. It ultimately conspires to destroy the Source wall, allowing the Anti Life entity to fully enter our universe and the Source to merge with it, becoming whole once again. At the end of Countdown, a battered Source wall is shown, with chunks of the wall floating in space. In Superman/Batman Torment, Superman is imprisoned within the Source while Desaad controls High Father's staff. However Batman is able to take control of High Father's staff. Superman uses the staff as an anchor and pulls himself through the source wall and out of the Source.