"Crisis on Earth-X Part 2" Has a Surprising Tie to 'The Flash' Mythos

"Crisis on Earth-X" might unite heroes from across four Arrowverse shows, but one of the crossover's villains ended up being pretty personal.

Spoilers for the first two hours of "Crisis on Earth-X" below!

This week's crossover saw the Arrowverse's heroes facing off against villains from Earth-X, an alternate reality where the Nazis won World War II. But as the heroes learned in the crossover's second portion, one of the villains actually hailed from Earth-1, and was none other than Eobard Thawne/Reverse Flash (Tom Cavanagh).

As fans will remember, Eobard was at the center of The Flash's Season One after posing as Earth-1's version of Harrison Wells. Barry and company eventually uncovered Eobard's evil scheme, and that Eobard was the one who murdered Barry's mother.

Eobard seemed to die in The Flash's Season One finale after his ancestor Eddie Thawne (Rick Cosnett) sacrificed himself. But Eobard would return to the The Flash again and again, including in Barry's "Flashpoint" timeline in Season Three.

Eobard then made his way to Legends of Tomorrow in its sophomore season, helping lead the Legion of Doom. While Eobard appeared to meet an epic end in Legends' Season Two finale, it's clear that a version of him from some point in time survived -- and found a way to Earth-X in the process.

So, what could this mean for the Arrowverse going forward? It's too early to tell, but it's clear that Eobard's story isn't over just yet.

"Crisis on Earth-X" is a four-hour event that will air on November 27th and 28th beginning at 8 p.m. and running for two hours each day; Supergirl (8 p.m. Monday), The Flash (8 p.m. Tuesday) and Legends of Tomorrow (9 p.m. Tuesday) will remain in their normal time slots but Arrow will move from Thursday to a 9 p.m. Monday placement for the week.