One of the most distinctive and powerful villains in the DC Universe is the Anti-Monitor. Created by Marv Wolfman, George Perez, and Jerry Ordway in the pages of Crisis on Infinite Earths, the imposing and seemingly unstoppable force has become a portent of troubles on a multiversal level. And though he's only made sporadic appearances over the years, he remains one of the biggest, if not the biggest threats to the DC Universe.
With his opposite number (the Monitor) confirmed to be appearing in the upcoming Arrowverse crossover “Elseworlds,” its only logical to expect the Anti-Monitor to also be involved. In fact, he's probably the chief villain of the event.
The Wall Of White
The Anti-Monitor isn’t a particularly complicated entity. The comics incarnation of the character has had his origins revised several times, but for the most part he's remained the same. Created as a dark counterpoint to the benevolent Monitor, the Anti-Monitor is entropy made literal.
While the Monitor was born on Oa (eventual home of the Green Lantern Corp), his opposite came to be on the anti-matter word of Qward. Deciding that he wanted to conquer/destroy everything in the multiverse, the Anti-Monitor went on to become the villain of Crisis on Infinite Earths, making himself quite possibly the biggest bad in the entire DC continuity.
Releasing a wave of antimatter across the Multiverse in Crisis on Infinite Earths, the Anti-Monitor is responsible for the complete erasure of worlds from DC continuity. Although he is eventually defeated, many heroes are forced to sacrifice themselves to stop him once and for all. This included versions of Superman, Wonder Woman, and scores of smaller characters like Aquagirl and Kole. More notable (and potentially threatening) for the Arrowverse are two of the most iconic deaths of the event: Flash and Supergirl.
We Must Save The World
That's right -- both Barry Allen and Kara Zor-El died while battling the Anti-Monitor. In Crisis on Infinite Earths #7, Supergirl went toe to toe with the monster, saving Superman from an attack, and even destroying some of the Anti-Monitor’s equipment while wrecking his armor. But it wasn't a battle she walked away from, as she was struck down by a sudden blast of energy when her attention shifted even for a second.
The very next issue featured one of the most iconic Barry Allen moments ever in the character’s sixty plus year history: His death. Alone and imprisoned on Qward, Flash manages to get free and use his speed to turn the energies of an anti-matter cannon back on itself, saving all of creation. But the influx of energy from the cannon is too much for him, wasting Barry Allen away even as he runs and mutters the mantra “we must save the world.” By the time the cannon is destroyed, his body has been reduced to dust in an empty costume.
Both of these deaths were monumental, and both actually subverted the typical revolving door of death in comics for years. Although there were other people who took up the mantle of Supergirl and Flash, it would take until 2005 and 2008 respectively for the actual pair to return to the land of the living. Which doesn’t bode well for the heroes going into “Elseworlds”.