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Nightmare Fuel: ReBoot
Comes in high doses in Season 3. Quick rundown:
Enzo, AndrAIa and Friskett playing a Mortal Kombat spoof, complete with an unbeatable User who slashes out Enzo's eye. In order to escape nullification (which is a majorNightmare Fuel concept in its own right) they have to entire GameSprite mode and leave Mainframe with the Game, leaving Dot to think that her only family was nullified. Oh, and Enzo and AndrAIa? They're KIDS.
The user's character fights a number of fighters and every time applies a finishing move of gripping a clawed hand around a victim's head, and then some crunchy Sound-Only Violence occurs and a ominous sting plays. Keep in mind that the most memorable thing of Mortal Kombat to many people was the icky fatalities, and Sub-Zero's ripping out the head and spinal column was widely considered the ickiest. This was all but stated as an obvious homage while Getting Crap Past the Radar, and innocent young Enzo is watching all these other CPU characters go through something too violent to show on-camera. He's actually somewhat mature about it, but then he's last seen laying defeated with the hand coming down over his face. And then the sting plays, as the camera cuts to AndrAIa and Frisket stuck spectating. Now that's cold.
Enzo and AndrAIa are stuck in a never ending cycle of violence as they travel from system to system via the games.
When they finally do return to Mainframe they find that Megabyte was able to take over and turned it into a warzone.
Phong is decapitated by Megabyte and his head is tortured into giving up the passcode needed for Megabyte to infect the super computer.
Enzo falling off of the rotating cards and into the many screens can be very frightening to a small child.
Los Angles in the episode is especially creepy, being full of abstract art (including nulls with Bob's face on them and imagery based on the works of Salidor Dali)
Megabyte, infested by the web creature. Black with silver highlights, the claws on his hands permanently extended, red eyes, and a serpent tongue.
There's also Hex's infested form, which involves much of the same, a black outfit with silver highlights, the serpent tongue, blue eyes that turn red as she approaches Mike and Scuzzy, presumably to kill them, and this is not only after we witness Hex being horrified as the web creature held her down and approached her, AND this is also not long after "Painted Windows" (only two episodes removed from it).
Basically, Hex could always be counted on for SOME form of Nightmare Fuel, intentional on her part or not.
One episode had a very creepy scene of Megabyte running up a vertical wall on all-fours, like some kind of Hell Hound.
The Funhouse, from the game itself to the Bad Future shown as a result of Bob and Dot losing it.
The Web Creature. It doesn't help that its full debut comes in the Paranoia Fueled episode"Trust No One" and that the damn thing looks like the unholy product of Cthulu and a vampire. Add in the fact that it drains the life from its victims with highly-visible "veins" that stick out of their skin and are organic enough to twitch and writhe in pain when severed... brrrrrr.
Dot's partial erasure from a magnet. She flashes like an X-ray skeleton, but with huge eyes. Overall it looks very creepy.
Daemon's infected all come off as very disturbing cult followers blindly following her word. It's a very chilling experience trying to talk to one of them.