Ever since the first big crossover ever episodes between Arrow and The Flash, the television event has always been a big highlight of the fall season. With every new show that joins the CW-Verse the crossovers just get bigger. And with the addition of the fully formed Legends of Tomorrow and the newly arrived Supergirl, this year's crossover is going to be massive. We know that the event will take up four nights of television, but that won't be the only thing that separates this crossover from previous ones.
The topic of the upcoming four-part crossover was brought up during Entertainment Weekly's Spoiler Room, where some new details were revealed. The writers are only just getting to work on hammering out all the details, but super producer Greg Berlanti was able to tease the threat that would cause all of these superheroes to work together. The crossover won't be a new threat for each episode, but one giant one that will play out over the course of the week.
The first year he's referencing is the very first crossover that began when they only had to worry about The Flash and Arrow. Taking up two nights of television, each episode dealt with one problem completely unrelated to the other. The second crossover took a different approach. It served as a sort of backdoor pilot for Legends of Tomorrow, not only setting up the mythology of Hawkgirl, but also introducing the immortal villain Vandal Savage. Savage was the unifying threat for that crossover, so expect something similar for this year, but on a much bigger scale.
We don't know the threat that causes these heroes to buddy up, but it'd have to be at least a little bit grander than "Immortal guy wants to kill hawk people." There's no new show to set up, so the writers have a lot more wiggle room. Supergirl currently exists in another universe, but whether that will still be the case now that's it's on the CW is unclear. If Supergirl does still live in another dimension, then the unifying threat could be one that threatens the multiverse. That'd also be a pretty good reason for the time hopping Legends to get involved, perhaps after seeing a dark timeline where everything has been destroyed? If not that, then the new Legion of Doom - a supervillain team consisting of characters connected to all four shows - is another good choice.
There's also the Flashpoint of it all to consider. Barry Allen essentially resets the timeline at the end of The Flash Season 2. There have been varying reports of how exactly this affects the other shows set in the same universe, but maybe this threat arises because of Flash's meddling in cosmic affairs. Then all the heroes are essentially teaming up to fix the timeline that he broke. According to Grant Gustin, Flashpoint is going to be a pretty big deal, so we'll have to wait and see.
The DC crossovers usually begin around episode seven or eight of the season, so keep checking in with CinemaBlend and we'll keep you updated with new information as it becomes available.