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It's just a few days until worlds live, worlds die, and the Arrowverse is never the same again when "Crisis on Infinite Earths" debuts on December 8. And in case it wasn't clear enough from the tagline, or from the trailer that seemingly shows Earth-38 being destroyed...well, Supergirl star David Harewood says that not only will "Crisis on Infinite Earths" touch of something very big for J'Onn J'Onzz the Martian Manhunter, but it will also "affect everything" in Supergirl's world. There are a lot of ways that could be the case, but of course fans are bound to zoom in on just a couple.

The first, and much less likely than others, is that Supergirl will die. She was, after all, destined to die in the Crisis before Oliver Queen got himself involved in the whole thing, and there's no guarantee that whatever he is planning will actually work. The second, and most popular, theory about what could happen is that Supergirl and her compatriots could be brought over to Earth-1 following the apparent destruction of their universe. Harewood was mum on either option, but did say that there are big things coming.

"I'll say there's an important change that's coming for me as an actor," Harewood teased. "It's going to come as a result of 'Crisis,' which is important for me as an actor, and for me playing the character. So I'm actually really looking forward to that, for the audience getting to see that."

That might be cryptic -- and Harewood is good at that, managing to avoid spoiling much of anything -- but what's even more intriguing is the way he talks about J'Onn's story during the Crisis and how it impacts his life after the crossover is done.

"The Crisis affects everything, and it's going to affect J'Onn. One of the wonderful things about playing J'Onn, and playing the Martian Manhunter, is understanding how deeply the philosophy of the character runs. It really is a deep character. It's been great fun to kind of explore it as an actor. One of his fundamental abilities, one of the Martian's fundamental capabilities, plays a very, very important role in the Crisis. It is specifically because he is the Martian Manhunter, that he's able to sit to survive the Crisis. But it will affect him in a fundamental way."

The event will bring together the heroes from multiple Earths to battle against the Anti-Monitor (LaMonica Garrett), a godlike villain who threatens to destroy all reality. In the comics, the story ended with the deaths of The Flash and Supergirl, and the destruction of DC's multiverse, leading to a single Earth with a complex history packed with hundreds of heroes.

The event will be the most ambitious thing DC has ever attempted in live action, bringing together characters from all six of the current DC Comics adaptations on The CW (Arrow, The Flash, Supergirl, DC's Legends of Tomorrow, Batwoman, and Black Lightning), along with characters and actors from the 1990 version of The Flash, the short-lived Birds of Prey, Smallville, Superman Returns, and the iconic 1966 Batman series.

“Crisis on Infinite Earths” kicks off on Sunday, December 8 on Supergirl, runs through a Monday episode of Batwoman and that Tuesday’s episode of The Flash. That will be the midseason cliffhanger, as the shows go on hiatus for the holidays and return on January 14 to finish out the event with the midseason premiere of Arrow and a "special episode" of DC's Legends of Tomorrow, which launches as a midseason series this year and so will not have an episode on the air before the Crisis. Black Lightning's midseason finale will have a "Crisis" tie-in as well, although unofficially.