With an example of a perfect anime crossover, it brings a lot of nightmares to the readers.
Underworld Saga- Basiliskmon. This huge snake can turn anything to stone with his fearful eyes.
Season 1: Wrath of Pharaohmon/Redux- Pharaohmon is one example since he's the main antagonist. Especially in his true form.
- His living brain at the climax is pretty gruesome as well.
Season 2: The Invasion of the RajitaSeason 3: Dawn of Chaos- Unlike the first two seasons, this brings ALOT of chaos since it's the theme of the story. It has raise the stakes of pure nightmares.
- The Valmarmon reanimated body parts are creepy enough to our heroes.
- Necromon. What does this guy do? He has the ability to bring dead people back to life as revenants and they are impossible to reason or kill unless Necromon dies. Seriously, he is indeed like Myotismon!
- The fact that he controls Kotori 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.
- 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 could make readers uneasy.
- The revelation of Sailor Charon as Chaos itself. Yes, THE ultimate sailor moon villain that is the cause of all evil forces.