Here is everything this article assumes about the film: the title, the date, and that Doctor Doom is at the center of it. That's what the studio has printed. Everything else below comes from the comics, where Doom's version of a doomsday has already happened, in full, on pages you can read tonight.
Act 1Start here: the incursions
The 2015 event the film's title gestures at begins with a physics problem, not a villain. In Jonathan Hickman's Avengers run, parallel universes start colliding, two Earths at a time. When two Earths touch, both universes die, unless one Earth is destroyed first. The smartest people alive form a secret club to handle it, and the club's minutes read like a war-crimes tribunal waiting to happen.
EIGHT MONTHS LATER. The heroes have run out of moral high ground. There are two universes left.
Act 2Then the multiverse dies, and Doom catches it
Secret Wars #1 opens at the final incursion: the Marvel universe and the Ultimate universe, the last two standing, grinding into each other over New York. Everything dies. And then page one of issue two is a psalm, because between issues, Doctor Doom stole the power of the beings who engineered the collapse and built a patchwork planet from the wreckage: Battleworld, stitched from dead universes, ruled by God Emperor Doom.
“This is the sum of everything. All that remains. And it is mine.”
For eight years of publishing time before that moment, Doom had been a punchline-adjacent tyrant in green. Secret Wars is the book that made him the most interesting character in the line: a man handed godhood who uses it to fix everything, badly, because even omnipotence can't fix what he actually is.
Act 3The origin underneath all of it
None of Battleworld makes sense without the origin: the Romani boy in Latveria, the witch mother burning in hell, the machine built to reach her, the classmate who pointed at a flaw in the math, and the explosion Doom has spent his whole life blaming on that classmate instead of the flaw. Books of Doom (2005) tells it best, and its final page is one of the great unreliable-narrator moves in superhero comics.
What the screen did
Whatever the film does with Doom in December, it has roughly 150 minutes to establish him.
What the page says
The comics had sixty years, and the two runs above are the distillation: a man who could rule everything, undone every time by the one face in the mirror he can't out-argue.
December 18 will answer what the film keeps. The pages already answer everything else.