The Oblivion Machine was built on earth 7 by the forces of the Gentry, under the direction of the Empty Hand. Their objetive was to work as a counter measurement to the effects to the Miracle Machine, used by several superheroes.

The Machine was finished with a final component, a crack on the multiverse caused when Barry Allen reached to the multiverse-2, leaving a scar, a crack in the bleed. Darkseid captured the crack and tried to use against the Empty Hand on earth 7, but it was that crack the last piece to complete the Oblivion Machine, which occupied the totality of eath 7 itself. With the crack at its reach, the Empty Hand activated the Oblivion Machine, unleaching the Greatest Darkness in the multiverse.

Real meaning

According to Grant Morrison, in an interview in Paste magazine.com, the Oblivion machine had a more funest meaning:

The Oblivion Machine in Ultra Comics; consuming comics is devouring the hours and the minutes of your lives, and you’re actually being vampirized by your entertainment media. That’s the ghost inUltra Comics, that you’re being devoured by the story that you’re consuming, because you could be out there meeting the girl of your dreams or flying out a microlight. It’s about the things that we consume, the pictures that we love to watch.

Grant Morrison, https://www.pastemagazine.com/comics/grant-morrison/exclusive-preview-interview-grant-morrison-unleash/