The ending credits for the beginning seasons. Accompanied by some utterly eerie and evil-sounding music, it first shows flames with the main characters as Tarot cards. Then as the flames begin vanishing and the credits come to a close, there's a very creepy Voice of the Legion.
In "Sickly Sweet", just to scare off a couple and real estate agent, Grim transforms a tiny, innocent flower into a giant, plant-monster... thing that begins to trash the ever-living hell out of Billy. It's made much worse as Billy screams "IT'S DEVOURING MY NOOOOSSSEEE!!!!!". The real estate agent responds with some mood-clashing Casual Danger Dialogue:
Real Estate Agent: Looks like we've got a bit of a weed problem.
Mandy's Sanity Slippage throughout the episode until she finally snaps at the end.
The so-called Christmas episode. Bad Santa? More like blood thirsty Santa. This must be the first time a show's Christmas special is scarier than its Halloween Episode, which is quite mild.
Nergal Jr. You don't want to mess with that kid. His "true form" is truly horrifying. There's also the shot of his tentacles coming out from weird fleshy holes in his back and head.
The "green-eyed kid" form he is usually seen on the show in isn't even his own original form, he stole it from the first person he met he met when he rose from the core of the planet. His original form is never seen by the viewer, but the original green-eyed kid is unfortunate to, and is outright terrified of it. The best part? The original green-eyed kid is neverheard of in the show again.
Little Rock Of Horrors with the Audrey II-like space rock that required Billy to get it human brains.
Brains, brains, it's okay! It doesn't matter just so long as it's gray.
But quickly made scarier through the fact it doesn't use any sort of alien magic or anything, all it promises is to be Billy's best friend. And there is no little boy or girl who doesn't want a friend out there. If Billy hadn't found it, would it have just preyed on some other insecure kid?
Dream A Little Dream had scenes based on the "Night on Bald Mountain" sequence from ''Fantasia'' but not as scary. It also had much more horrifying segments for Billy and Grim's nightmares. Billy initially is overjoyed to be in a seemingly tranquil, beautiful and lovable place that Tastes Like Diabetes. Then he finds the cute little baby rabbits that transform into nightmarish monsters that chase him. However, Billy ends up running right into a horde of killer rabbits. Grim, meanwhile, dreams of the unfortunate reality he's stuck in - he's Mandy's eternal slave, and at the end is beaten up by an army of Mandy clones.
Nursery Crimes featured a tale from Mandy that had Billy cast as Humpty Dumpty... who falls to his death and shatters, and his remains are eaten by all the king's horses, men, and villagers for days. It's quite terrifying to think Mandy has that much hate for a person in her heart.
Then we get the already somewhat creepy Pinocchio trying to eat Billy's flesh. Mandy saves the day by carving Pinocchio into a chair, and Pinocchio is seemingly killed when the witch from Hansel and Gretel crushes him under her weight.
And finally, the ending. Mandy is trapped inside the fairy tale book, with Grim having fallen asleep on them. Mandy only quietly swears vengeance on us all.
In "Chocolate Sailor", Billy turns completely into solid chocolate after eating too many supernatural chocolates. If that isn't enough, Billy even begins to eat himself, saying to Mandy that "...it's not that painful" at one point, and continues to do so, reducing himself to "his big, chocolate head" near the end of the episode. At the episode's climax, the Chocolate Sailor gives Billy the antidote, but with a twist: it's in a box of assorted chocolates and Billy has to pick the right one. Since Billy is stupid incarnate (the antidote bar is called Antidote), he eats the whole assortment and promptly explodes. The episode ends with Mandy and Grim eating hot fudge sundaes, with them saying Billy is in a better place. The bottle of hot fudge syrup they have has an illustration of Billy on it who says "I like chocolate!"; implying that they scooped up Billy's remains and dumped them into a chocolate syrup (and to add that Grim and Mandy are eating hot fudge sundaes...)
The Cthulhu episode. The real Cthulhu (the one playing golf is really one of his Star Spawn) shows up and eats Billy at the end, meaning they failed to stop him.
Pud'n's part.(shudder) What really drove that one home is how realistic the bunny was drawn. When Pud'n's holding the cute bunny and it lets out a terrifying shriek as its face contorts. Cut to Pud'n's face looking like he pooped himself. And when the bunny rises out of the fire of the destroyed truck in an obvious Terminatorhomage, complete with its insides showing and says "Of course I forgive you, Pud'n! I forgive you because I love you... And I love you, to death." The epic Mood Whiplash (the truck had just blown up from having its cargo, agiant pillow, hit with thecreme filling from an eclair) really made the moment scary. It does just turn into standard cartoon violence for parts of it, but the fact that it goes from that to the said quote is horrifying. That voice is just awful.
It was disturbing that Pud'n was searching through the garbage for candy.
The character of Pinface considering his voice is done by John Kassir (you know him as the voice of the Crypt Keeper in Tales from the Crypt.
Most of "Who Killed Who?" is absolutely terrifying, especially if you haven't seen all of it. It's one of the episodes that use Scare Chords more often than the normally cartoonish sound effects, adding to the horror even more. That's not even counting the Twist Ending, where it's revealed that Mrs. Doolan really was a ghost.
Mandy: Mrs. Doolan beat Grim. She beat death.
Also, the possibility this episode is where Bleedman took inspiration for Grim Tales from Down Below. Mandy did the same thing Ms. Doolin did. Only rather than just defeat Grim, she married him, thus taking the power of Death for herself.
From the Billy and Mandy's Big Boogey Adventure movie. The main cast and Irwin are put into a sort of dream loop by Boogey. Mandy wakes up on a beach by the River Styx and says "Wow, I just had the craziest dream...". Billy (off camera) then says "Is it the one where you couldn't save us?" We're then treated to a FREAKY looking creature that has Billy, Grim, and Irwin as part of its body as it wails in a distorted voice, "Where are you, Mandy?"
In one episode, Grim falls for a goth girl and struggles to tell her that he is, in fact, the Grim Reaper. So how does he show her? By disturbingly rising from the ground while fire forms around him whilst he randomly takes on this slinky look akin to Slender Man with a horrifying frown throughout.
In one of Mandy's intro quotes, she just walks on screen, points at the viewers, and lets out a demonic screech. This scene, or at least one like it, was actually a Shout-Out to the alien screech from Invasion of the Body Snatchers.
Mandy smiling. Not her evil smiles, but a genuine grin. Considering her character, it's extremely unsettling.
"A Dumb Wish", in which Mandy wishes that everyone in the world except her would go away.
The ending to "Scary Poppins", which shows Mandy apparently controlling the minds of everyone in Endsville.
Mandy: This is my world. I'm just letting you live here.
The trailer to "Wrath of The Spider Queen". The Spider Queen's singing as Grim says the title of the special(with him also giving a good punch to the fourth wall in the actual special) is particularly unnerving.
Billy's nightmare in "Tickle Me Mandy".
The ending to "Spider-Mandy", where everybody in town has become a mutant spider creature. This proves to be in-universe nightmare fuel to Billy.
Spider Gladys: Do you want breakfast...BEFORE I EAT YOUR HEAD!?
In "The Halls of Time", Billy and Irwin start to mess around with a few hourglasses that control the lifetime of all people in the show, and when Billy accidentally let fall one of those, it ends up killing Maxwell Atoms, the creator of the show.
Not only that, but Irwin kills his own brother by throwing his hourglass away.
We get to see Billy's father choking to death, but it's immediately revived by Billy putting his father's hourglass upside down.
At the end, the two end up accidentaly putting their own hourglasses, and the ones from Grim and Mandy, upside down, making them grow up backwards until all of them become fetuses, and die. The worst part is that Grim is happy watching Mandy dying before his eyes. But at the very end of the episode, Grim is unable to put his own hourglass upside down, then gives up, waves at the camera and dies.
Hoss' backstory, where he was constantly one-upped by a guy called Kyle, whom he now keeps chained to a radiator in his basement, would be this if Kyle wasn't a traffic cone.