Kaiba's sheer obsession with dueling Atem is so great that he transcends the boundaries of dimensions to see him briefly, almost dying in the process.
A lot of what Sera does and says is creepy. She has knowledge beyond her years, and claims to hear voices from the netherworld.
Sera challenges Kaiba to the technological equivalent of a Shadow Game, which would kill him if he lost.
In the official translation of part 2 Kaiba vows to reach the afterlife and put an end to the dead king. Whether he's simply being dramatic or means it literally, his obsession is razor-sharp and terrifying.
Sera manipulates Kaiba by playing to his insecurities about the Pharaoh and his own skills, saying he'll never become the duel king—but if he links with the voice she hears from the netherworld, perhaps he can do something. Kaiba attempting to do so nearly kills him, which is what she and Aigami were planning to begin with. The only reason Kaiba doesn't die is due to Mokuba cutting the power.
The third English trailer has a very ominous tone, showing Aigami disintegrating Kudaragi's gang and the characters discussing it.
Duke: I saw something online about 50 people vanishing all at once! Anyway, enjoy your lunch. Bakura: But how can we knowing someone or something is making people vanish? That someone could be watching us right now?
Even worse is a line Aigami has later on over shots of a character's hands disappearing.
Aigami: There will be nothing left of you!
The movie opens with shots of multiple parallel Earths disintegrating.
The trailer features shots of Aigami being engulfed by shadows, as well as kids under mind control with glowing, golden eyes.
According to Aigami, thanks to Kaiba nobody in Domino City can have citizenship without a Duel Disk. Transcend Game says his latest dueling system collects data from the brains of the Duelists themselves, and he can track anyone in the city by their disks or brainwaves. Suddenly jokes about Kaiba being a dictator aren't as funny.
Aigami's Quantum Cube that originally belonged to Shadi can trap people in other dimensions. Anyone not part of the dimension originally will die. It can also outright erase people, who dissolve head-first.
The Cube itself, really. In the wrong hands, the power to travel between dimensions, and to the Afterlife itself, could be exploited.
Jonouchi is trapped in an alternate dimension that slowly dissolves around him, nearly killing him.
Bakura experiences a flashback of how he got the Millennium Ring, making a horrifying face as the Ring fuses into his chest and protrudes from under his skin. It's followed up by Yami Bakura killing Shadi. Making it worse is that the Ring/Yami Bakura also killed his father in the dubbed movie. Whether the father dies in the subbed movie is left ambiguous.
For those who have read the manga, the shots of the Ring's points turning towards Bakura's chest likely gave them a scare, as the prongs gorily pierced into his chest during the Monster World arc. Pegasus's flashback also shows the Ring burning the unworthy to the bone from the inside out, leading to fears of Bakura's father meeting the same fate.
The Millennium Ring is so evil and powerful that even though Zorc and Yami Bakura are gone it corrupts and possesses people.
At first, Aigami's powers utilize a golden light. Once the Millennium Ring corrupts him and Mani, his powers turn a shadowy purple instead.
At the climax, the Millennium Ring possesses Aigami and he turns into a horrifying monster with cubes for eyes. He proceeds to call a Shadow Game, defeating Kaiba and nearly defeating Yugi before being stopped. By the time the final duel's over it's implied the patch of space where they were dueling was the only thing left on Earth.
At the end of the movie, Kaiba successfully manages to enter the afterlife, hellbent as he is on dueling the Pharaoh one last time. The kicker? We don't know if he's dead or not.