Dyson's Dawn, better known as the Dyson Sphere, was a bubble universe that was used by multiple different groups.
It first became active after an older version of Lady Aesculapius came into contact with Dyson, the physical manifestation of the Sphere. The two used it to entrap people from other universes, most outside the Dawns, in order to lure people who saw them as stories into the Sphere and steal their life-force. Lady Aesculapius and Jason Jackson, however, were able to intervene and stop the plan. (PROSE: A Possible Prelude to an Ending)
At some point J.P. Barnum purchased the rights to it from Dyson, allowing him to turn it into a vast recording studio. The dimension began to collapse in on itself once Barnum was removed from it, eventually winking out of existence. (PROSE: We Believe What's On TV)