First Appearance: The Uncanny X-Men and The New Teen Titans
The Source Wall is a physical wall that lies on the edge of the known universe, in the Promethean Galaxy.
Beyond the wall lies what is thought to be "the Source"; a cosmic essence or being that is the "source" of all that exists. However, in the events that surrounded the death of the New Gods, it was revealed that only "the Bleed" exists behind the wall; seperating the multiverses from each other.
The wall is theoretically passable. However, all those who have tried have been inevitably trapped in it. Over time it has been made up of the bodies of would-be conquerors and curiosity seekers from all across the universe. It seems that humans and other beings, such as angels, view the Source differently when they pass the Source Wall.
One of the few known persons to have passed the wall and entered the Source was an amalgam of Zeus, Odin, Ares, Jove, and Highfather. Five others are known to also have passed the Source Wall at various times: Barry Allen, the Spectre, Metron working with Swamp Thing, and Lucifer Morningstar.
It is revealed that the 52 universes are each surrounded by a Source Wall, with "the Bleed" existing between the source walls of the various universes. The Monitors 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.
During his travel of the multiverse, Superman-Prime operated out of a cave within a Source Wall, 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 has been attached to and removed from the wall multiple times. Throughout his 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.
Following the Death of the New Gods, a second Source Wall was created to store the souls of the slain New Gods.