Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: YuYuGiDigiMoon
With an example of a perfect anime crossover, it brings a lot of nightmares to the readers.
Special Gaiden: Underworld Saga
Basiliskmon. This huge snake can turn anything to stone with his fearful eyes. And crosses the line by poisoning and murdering Kotori Ayami.
Season 1: Wrath of Pharaohmon/Redux
Pharaohmon is one example since he's the main antagonist. Especially in his true form, and going in his full power.
His living brain at the climax is pretty gruesome as well.
BelialMyotismon's Nightmare Illusion on the Sailor Senshi.
Season 2: The Invasion of the Rajita
Kaiser Ghidorah. This guy is nearly unstoppable even the combine forces of the heroes at this point.
His true form as a kaiju counts.
The Rajita's invasion on Earth nonstop.
Kuiiza's nightmarish torture on Joey Wheeler in converting him to the dark side.
And how about the fact that Dark!Joey transforms into the Red-Eyes Darkness Dragon that can frightened a T-Rex.
The concept of Orichalcos, which turns out to be alien origin of the Rajita.
The return of the Great Leviathan. Very intensely frightening for its colossal size. Plus, it can speak.
Movie: The Taiyoukai Awakening
The Taiyoukai as a whole, especially Kiryu.
Their combined form at the end counts as well.
The fact that the demons' twisted plan on using human females to breed half breed warriors is far disturbing.
Season 3: Dawn of Chaos
Unlike the first two seasons, it has raise the stakes of pure, chaotic nightmares.
Basiliskmon may be scary, but his new form KingBasiliskmon took it Up to Eleven.
DexInumon, a dark corruption of Inumon infected by the X-Virus. It is a hellhound wanting to burn everything it his path.
The Black Panther, a darkness spawn from Phillipe's heart. For in the shadows, he might lunged and devour your flesh.
The juggernaut is activated in Chapter 57, and Janyu Wong, Charon and Prophet are nowhere to be seen.
Valmarmon himself. His reanimated body parts are creepy enough to our heroes.
ValWingmon briefly possessing Jeri gives the chills.
And the Devil of Digimon has return in Chapter 54. His titanic size gives a frightening appearance.
In chapter 59, Valmarmon becomes scarier by transforming into a kaiju that looks like Destoroyah.
Necromon. He has the ability to bring dead people back to life as revenants and they are impossible to reason or kill unless Necromon himself dies.
The fact that he controls Kotori Ayami/Brunhilde is disturbing, especially when she unleashes her dark valkyrie powers.
If worse comes worse, Necromon also revives IceDevimon and Pharaohmon's minions, leaving Beelzemon to deal with the zombies.
His evolved form, Lichdramon, is just as scary. A dracolich filled with Dem Bones.
Chapters 31 and 52 shows NeoDevimon in his full power. He removes his mask and reveal a horrific looking face.
The Moon of Valmarmon. That is all.
In Chapter 55, Valmarmon uses his magic to fuse Yamidramon and Atolmdramon to become the nightmarish Doomsdramon as his hellish steed. It's roar doesn't help.
In Chapter 58, Madoudramon Titan Mode opening Digital Hell release the Digimon Villains as zombies. Unlike Necromon's, they are truly mindless as they march in droves. Diaboromon and his clones are part of the undead as well.
In Chapter 59, all the Diaboromon killed in the Real World have their data fuse into Armageddemon.
Gamera is twice as intimidating like his father, Kaiser Ghidorah. Heck he even has his memories!
In Chapter 55, Gamera and the Neo-Rajita are planning to terraform the Earth.
Chapter 57, thanks to the power Gamera stoles from Alphamon, he can rebuilt himself at his own will.
Chapter 59, he finally gets the Romulus Source and becomes a giant.
Helena's abuse by Kitoss instill fear and hatred at the same time. Of course by the time Helena recovers, she briefly becomes cold hearted that could make readers uneasy.
The concept of Meikai.
The Demon Priests in general.
Chapter 48, Karasu is possessed by a symbiote that makes him become Venom.
Long transforms into his Taiyoukai form in Chapter 49 that looks like a hydra.
Arago's revival by taking over Kohana's body.
King Enma's death by Arago's hands in Chapter 55. Enma also saw haunting ghosts of Queen Serenity and Odin mocking his pride. Afterwards, Arago and the Demon Priests take over Spirit World and turn it into a gothic hellsphere.
In Chapter 57, they freed old YYH Villains and begin their dominance on the human world such as the Makai Insects and Cultivated Humans.
In Chapter 58, Arago even revive the Taiyoukai Eight as well! Plus, Long assimilates with Rando, Suzaku, and Murugu to become OmegaLong.
Professor Frank and Company
The Paradais Knights' way of making dueling holograms real and cause injuries to their victims. Even Professor Frank is shown to be a psychopath at times.
They even blow up Kaiba Manor. Before that, they attempt to fake it in thought of doing it on Kaiba Corp.
The Earthbound Immortals. And Professor Frank summoning the Egyptian God Cards.
Sailor Charon and The Prophet are some of the creepiest people you might encounter. The rebuild company was once Dartz', but they control the shots now with chaos.
Chapter 18 shows the Paradais building's true appearance is an alien hive that's similar to Alien. There's also this painting of a dark creature, that Jeremiah said it's his favorite, when the seal cracks, it'll destroy everything. Also, the EYE that's briefly on the floor.
Sailor Charon playing mind games on Sailor Sedna in Chapter 28 nearly sends her to depression.
Chapter 32 is when Paradais sends the heroes to different dimensions.
The revelation of Sailor Charon as Chaos itself in Chapter 48. Yes, THE ultimate sailor moon villain that is the cause of all evil forces.
The Prophet and Jeremiah are one in the same in Chapter 51.
At Chapter 52, Paradais takes over HYPNOS Headquarters to spread their network. Comes A LOT of dark crystals appear across the globe.
Also, Sailor Charon proudly reveals to Cain that she killed his wife (Mei). Her face then turns into 'Mei' and kisses Cain before slamming him. "How's that for a mindfuck?"
Chapter 55, these quotes: "The eve of chaos has arrived." "Indeed, my love, and these final days will be eventful."
Chapter 57, Paradais conjured defenses to hold off the Tamers such as blobs that look like evil Cherubimon's face, and a visage shaped like Charon that fire energies of crystals. That reminds some readers of the D-Reaper.
Chapter 59 reveals that they survive from the Juggernaut!