The revelation of who Earth-X Prometheus is: Tommy Merlyn.
Ollie tries to get through and it looks like it's working...then Tommy just snorts on how "weak" this world is and takes a cyanide pill.
Even worse? When Ollie tells Tommy-X of what happened to his Earth-1 doppelgänger, the other man just shrugs it off. Think of how twisted this Earth is where these two friends are not only twisted and evil, but see one killing himself as a point of pride.
Wells shows the gang footage of Earth-X's history and dominance and noting how it was exactly like Earth-1...except for Martin correctly guessing the Nazi's invented the atomic bomb first. Adolf Hitler lived until 1994 to see the Reich take power. They're all affected but clearly, Felicity is shaken the hardest, turning away at the sight of a world that's a nightmare to anyone who's Jewish.
We knew the basic idea of the crossover would be evil versions of Oliver, Kara, and Barry, many speculating Barry's Earth-X doppleganger to essentially be Savitar without the metal suit or scarred face, and in the Reverse Flash's outfit. That is, until pictures of Tom Cavanagh in the suit surfaced; while many still thought it'd be an evil Barry, most correctly guessed the return of the Arrowverse's resident temporal cockroach and Barry Allen's personal boogeyman, one Eobard Thawne, once again wearing the face of Earth-1's Harrison Wells. And of course, unlike the time remnant with Thawne's true face (Matt Letscher) that tormented the Legends, this is the real deal, complete with Hand Wave about where in the timeline he came from. He is bad enough to begin with. Throwing in with Nazis from another world just makes it worse.
And for that matter, Earth-X's Oliver Queen and Kara Zor-El. One is the Fuhrer of Earth-X, the other is his wife and essentially a Kara who sees herself as a god above humans. If you thought "our" Kara under Red Kryptonite was bad...
It's bad enough to see Earth-X Quentin Lance as a cold Nazi, but worse is when he reveals that Earth-X Sara was also bisexual...and he killed her because of it. Worse still is that while Nazi Quentin notes that Sara looks like Aryan perfection, it's not clear whether he recognizes her as identical to his daughter, suggesting that Earth-X Sara didnt live long enough to look like ours.
Earth-X Felicity is a concentration camp prisoner, complete with star on her shirt.
To anyone who knows anything about the Holocaust, the Earth-X concentration camp: the gate with "Arbeit Macht Frei" ("Work Sets You Free" in German, used in the entrance of several death camps) over it, the nearby trench, and haggard and shaved prisoners in striped uniforms with cloth badges. There is no question what happens at that facility.
Seeing Stein, a Jewish rabbi, being gunned down by Nazi soldiers is nothing short of stomach-churning.
Thawne nearly putting a vibrating hand through Felicity, gloating that while his history books are filled with stories about heroes like The Flash and Green Arrow, no one has ever heard of her. And he puts on his glowing red eyes and Voice of the Legion while doing it, presumably just to make it more terrifying.
The implication that Thawne is working with the Nazis purely For the Evulz (since he never displays any obvious Nazi-like sentiments like his fellow Reichsmen) arguably makes him worse than the Nazis themselves. He is willing to sellout his own Earth to invaders seemingly for no other reason other than to spite Barry and people he cares about.
The Nazi troops are on the metal grate for the portal to protect it as a wounded Stein reaches for the switch. Realizing what's about to happen, their commander tells them to fall back but it's too late. As the portal activates, they're all held in place, shaking as if electrocuted and then blasting into pieces.
Thwane coming terrifyingly close to cutting Kara open and loving it. He's only stopped by the arrival of the Atom shrunk and holding back the blade before growing to knock them back.
At the end of their fight, Barry reluctantly lets Thawne go, knowing that killing him then and there wont stop other incarnations of him from coming. Before he takes his leave, Thawne casually threatens to steal someone elses face for when he comes back.