Before Superman's power. Following his battle with Heaven's hunter Angela, Spawn finds himself in Hell's Seventh Sphere. Inside he discovers an arcane prison that is later revealed to be full of famous comic book characters. When his powers don't seem to break through, the imprisoned characters offer their power, one of them being one of the most famous and recognized characters of all time, Superman.
When Spawn tries to operate Angela's abandoned lance following their battle, the weapon unleashes some kind of failsafe that causes Spawn to disappear, teleporting him to Hell where he enters the Sphere known simply as Erebus. He discovers it to be full of an ancient yet still stable prison across from a line of prisoners who are bound with hoods over their faces. Realizing they have a visitor, the prisoners reach out for Spawn, not out of hostility but with the hope that he'll be able to free them. Although only one arm is visible, it is clear that the prisoners are well recognized DC and Marvel characters, both heroes and villains trapped for an unknown period of time.
In Spawn #10 by writer Dave Sim and artist Todd McFarlane, Spawn uses the full extent of his powers, in an attempt to free them, throwing any caution about the consequences to the wind. The former Al Simmons' mind is flooded with hundreds of thousands of voices offering their power in hopes that it'll be enough to break through. One of those voices, although his face isn't clearly shown, belongs to Superman, whose voice both calms and inspires Spawn. The hero called "He- Who-Came-First" offers his power and convinces Spawn its now or never, ending their brief interaction with a wink and smile that Spawn reciprocates with a respectful nod under his mask.
Even though the story is considered non-canonical within the Spawn universe itself, it doesn't make Superman's appearance nor his transference of power any less significant. Although he couldn't be portrayed completely, the issues creators drop enough hints, both verbally and artistically, to make it clear whom Spawn is talking to. Not only is Superman one of DC Comics' greatest and most recognizable heroes, but he's also one of the comic genre's earliest superheroes. His powers essentially make him a god within his world so adding that to Spawn's influx of unfathomable power would make the Hellspawn one of the most powerful character in comics, period.
Although Spawn's power are at levels he could have never imagined, his explosive efforts do nothing to destroy the prison or release his captives. The formerly jovial Superman resolves himself to his cell where he ominously utters, "Doomsday," a reference to both the creature who infamously killed him and the futility of hoping for freedom. The writer Dave Sim, whose most popular work Cerebus also makes an appearance, uses the comic as a criticism about creators' rights within the comic book industry, with the hooded figures representing the creators who sold their rights for money and now suffer as they hear their creations lamenting in captivity. This wouldn't be the last time Spawn would cross over with other DC characters (though he has yet to have an official Marvel crossover), but one can only imagine what would happen if he should meet Superman again, and if they would be foes or allies.
