The thought of whatever happened to Yuya after presumably going into Helheim Forest.
Helheim Forest itself is rather creepy. Everything comes in inverted colors, there are Inves monsters roaming around at every turn and people even get a sinister temptation to eat the fruits off the trees and plants.
The Unknown Girl who appears to give cryptic warnings to the Riders saying that You Can't Fight Fate, they'll have to fight to the bitter end and there's no turning back. Mitsuzane's particular response saying that he'll suffer and bear the burden as long as Mai notices him doesn't mean good things down the road for the kid.
The most she can tell Mai about the fate awaiting her is to boldly encourage her to leave town.
Any time Mitsuzane reveals his ruthless side. It's quite jarring to see how quickly he can go from a sweet kid to a manipulative cutthroat who will do whatever it takes to get what he wants. It's even more jarring to see him and Sid actually getting along because of it.
#4: Zangetsu's ruthless Curb-Stomp Battle against Gaim. The fact that a powerful Armored Rider comes at Kota with the intent to kill him is enough to break our hero. It gets worst when you consider that Kota could've been killed in Helheim Forest and no one would know about it.
Zangetsu:This world is filled with evil. Evil beyond purpose or reason.
#6: Oren. Just Oren. In his debut episode, we learn that he's a flamboyant baker. Then we learn the fact that he's a retired solider meaning that he's killed people on the battlefield before. And let's not forget that he has sadistic and homicidal tendencies as well. While he provides a certain amount of comedy and fights for the thrills and entertainment, we can't forget that he nearly attacked the defenseless dance members of Team Gaim just because he was annoyed with them. It's scary to think how much damage he could cause if he got serious and actually went out to kill people.
And to make matters worst, this guy has gotten a hold of a Sengoku Driver, he has no problems with beating Armored Riders (who are teenagers) into violent submission and the Yggdrasil Corporation is perfectly okay with that.
Granted, Takatora is violently against letting Oren keep the Driver and immediately recognizes the danger he poses.
#8: Near the ending of the episode when a man is suddenly dropped in an allyway and the camera pans up to reveal an Inves hanging off a wall. Then it's followed by Mitsuzane's Wham Line.
Mitsuzane:Mai got stranded in the forest, right? What if an Inves came from there and got stuck in our world the same way?
#9: The first minute of the episode. A worker is walking around a factory and spots the fruits of Helheim growing. Picking one of the fruits off, he goes into a hypnotized trance and says "It looks... so... delicious...". Later in the episode, Kota finds the hard hat of the worker covered in leaves. Considering recent events, it's highly likely that the worker became the Komori Inves that terrorized the town.
Also, when one of the Yggdrasil workers fires his flamethrower at Kota and Mitsuzane. You see the flames coming right at you in first person.
#11: Kota's speech at the end of the episode.
Kota: Looking back on it, that was when the gears were starting to turn. But... we really didn't realize anything at all. We didn't know that our fates were already written in stone. We just wanted the power to reach our dreams, we thought that would help us grow up. That's what we all believed, at least. But... you can't wish yourself into growing up. You grow up when you can't be a child anymore... In the endless war that has yet to come... we would discover that for ourselves...
#12: The debut of the Energy Lockseed for Zangetsu. And he looks like he's ready to kill!
Another from the twelfth episode is when it's implied that Yggdrasill has spies everywhere. It's the waitress at Drupers. She seems to listen in on Kota and Micchy’s conversation, suddenly appears behind them and keeps throwing them eerie looks.
Mitsuzane channeling his Kureshima side again, but this time his unsettling behavior seems to last through the entire episode. Not only does he try to pressure Kota into giving up his belt, his whole character development seems to head backwards when he twice chooses to just watch innocent people get attacked by Invase rather than use his own Sengoku Driver, and even tries to force Kota to not help.
#13: People injured by Invase are getting sick due to some sort of bacteria leaking off of the monsters...
Yggdrasill fully anticipated this. And while the members of the inner circle casually discuss their plans, people in the hospitals are screaming and suffering. The contrast is rather off-putting.
Hase holding a Helheim Fruit and looking like he's gonna eat it in the preview caused some frights considering the wild mass guess that the fruits turn people into Inves... AND IT'S CONFIRMED! Worse yet, as of now there seems to be no way to transform people back. And considering that our heroes only ever cut Invase down, things are not looking good for Hase.
From the looks of things, Hase's on the brink of falling into despair, and given last season, there's the haunting fact that even if Hase didn't eat that Lockseed, he would've turned into a kaijin anyway.
Hase's half Invess form. Imagine an Invess with a human face.
Also, as it turns out, the Komori Inves from the ninth episode is most likely the construction worker who found the premature Lockseed at the beginning of the episode.
Recall back in episode 1, Kota got hurt pretty badly by the Inves before he finally transformed, and there's the fact that only a SINGLE hit from an Inves can make people contract the disease.
People can breathe easy, since according to the Wikia, The Inves know about the disease and can control it, only choosing to infect others when they need more immature Lockseeds... Which most definitely means Rat is infected.
Everyone relax. Rat's not infected, just beaten the hell up. The doctors gave him the all clear.
#14: It gets far, far, FAR worse. Look at Hase's Inves form, Hekija/Bixie Inves. It looks eerily similar to the Byakko Inves from the first episode with the green and white "skin", as well as it's right arm having a claw and it's left being more "human." If this isn't a coincidence...then we finally know where Yuya went...
To that dimension where defeated kaijin go. Yuya ate a premature Lockseed, and became the Byakko Inves. As in, the same Inves Kota killed in his first fight as Gaim.
#19: The title of the next episode in the preview gives off a very scary vibe. "The Invasion Begins with the End of the World".
#20: Kaito shows a very disturbing side to him. He declines the offer to join Yggdrasill despite them showing him, they really were saving the world. He doesn't decline for moral reasons, or even revenge. He declines as he REFUSES TO SAVE THE WORLD. He thinks a world that needs lies to survive should be destroyed. He even calls the invasion a good thing, thinking that only those that can fight should survive.
And this is followed by Sid, Ryoma, and Yoko laughing and revealing that they think exactly the same way. Not only does this confirm that these three are completely insane, but then you realize that these are the people Takatora is relying on...
In a previous scene, we see an Inves victim growing vines from his wound. While we've already seen other infected before, this is the first time we're actually treated to a shot of seeing the disease grow.
What about the story behind Helhelm itself? As Takatora puts it best: "Evil without reason or purpose".
And that story is the seeds came from a different dimension and completely took over the dimension that is now Helhelm. Every animal is now an Inves, that includes the humans. Takatora states Earth has ten years before the same thing occurs
And guess what? It's happening in the real world too, in the form of invasive species. No, not to the extend it will cost human lives but invasive species had caused damages to native ecosystem and some even damages human infrastructure.