Spoilers abound below.
- In "Free Cake" when Skips is sentenced by the Guardians of Youth to shrivel up into dust and disappear forever. Cue Skips rapidly aging, then collapsing to the ground slowly dying, his skin turning into dust while his skeleton stays intact. He even tries to speak, but can't, only groan in a whisperly voice. But he gets better once Mordecai and Rigby make an Equivalent Exchange by sacrificing the cake.
- The climax in "Grilled Cheese Deluxe" when Mordecai and Rigby end up preventing the anti-matter from destroying the city by sacrificing the grilled cheese sandwich only to be zapped by the anti-matter and get distorted and stretched in freaky ways. At one point, Rigby gets turned inside out. Mordecai got stretched out to the point where he was barely visible. And let's not forget about Rigby being ripped in half and being put back together in a matter of seconds.
- The demonic taxi that haunted Rigby's nightmares in "Ello Gov'nor" was actually pretty frightening at times, especially during one of Rigby's dreams where it crashes through the window, baring its teeth and screeching, accompanied by the freakiest Scare Chord ever. You can't blame Rigby for getting so freaked out by that thing.
- Let's not forget the second part of Rigby's dream where he sees the taxi and runs to Mordecai for comfort. Mordecai is then revealed to be the taxi. Oh and it rips through Mordecai's head like it was paper. Yick.
- When Mordecai sends the microwave barreling toward the end of time with him and Rigby on it in "It's Time", during their argument Mordecai pushes Rigby off of the microwave and he disintegrates into nothing but a skeleton with eyes before shriveling into a bunch of dust.
- Peeps is giant eyeball that will always be watching you, everywhere you go, until you die. Mordecai challenges him to a staring contest and if he wins, Peeps must leave, but if Peeps wins, he gets their eyes... and he will do so using an ice pick and an ice cream scoop. Oh and he has a rather creepy voice and can grow multiple tentacle-like eyeballs.
- The Season 3 premier episode "Stick Hockey" has a flashback where Benson and his student, who looks exactly like him but is orange, are in the stick hockey championships. Benson wins his round, but the student gets his head decapitated by the game sticks. Benson picks up the head of his student as he watches his student's final moments slip away. He then says, "Master, I thought you said that this was going to be so much fun!". Immediately afterwards, he dies.
- Muscle Man being flayed alive, Mordecai being decapitated, Skips being incinerated, and High Five being shredded. The Halloween episode is pretty grim.Muscle Man: I told you I was ripped.
- There's also that scene from the end of "Death Metal Crash Pit" where the RV crushes the ghost band, causing an explosion that skins, burns, and melts the entire audience, even showing eyeballs hanging out of their skulls! It even kills Muscle Man and Hi-Five Ghost.
- "Diary" had Skips perform a ritual right out of Raiders of the Lost Ark with the groaning spirits, and when Rigby messes it up the spirits form into a giant close-up of Margaret's face, twisted into look of pure hatred and gives a bone-chilling shriek.
- In the Terror Tales of the Park episode this season, Margaret has a pretty fucked-up sense of humor when it came to her ghost story; the story involves Mordecai, Rigby, Margaret, and Eileen needing a ride to a movie theater since Benson won't let them use the cart on account of pranksters TP'ing the park. Eileen calls a cab but a strange bus called "The Party Bus" pulls over in front of them before the cab showed up, they then board the bus and are greeted by a bus driver in mummy costume and join the other party-goers on board. While they were having a good time as they waited to arrived to the theaters, they soon found out the party-goers were aging to dust, and so were they. Mordecai and the others desperately tried to escape the bus before they die, but the driver, who turned out to be a skeleton, persistently prevented them from getting off the bus. Mordecai managed to reverse the bus' aging effect on them by shifting the bus in reverse, but it worked too well as the reversed aging effect made them so young they reverted to being children. They finally escaped through the bus' escape hatch, which at this point they've became mere infants, and jumped off the psycho bus. Unfortunately it was too little too late as they all de-age to children to the point that they disappear, and this horrifying tale ends with the bus driver laughing menacingly as the bus fades way to nothingness. Even Rigby found that story pretty fucked-up.
- In "Guy's Night", Pops is send to the dimension of the Milk People and meets other guys who also drank an entire milk galloon as part of the Milk Challenge. There, they are attended by the Milk People, who serve them glasses with milk, but they soon realize that the Milk People are horrifying creatures who have no eyes but a mouth with vampire-like teeth who want for all the guys to remain there with them. While Pops and most of the guys manage to reach the elevator and get back to reality, one of the guys is bitten and transformed into a Milk Person and other is dragged by the Milk People to the same fate. That's when you realize that now as Milk People, these normal guys have lost their humanity and will now be forced to spend all eternity there.
Season 8 (Regular Show In Space)
- The villain of this season, Anti-Pops. Feared all over the universe, has a massive fleet at his command, and he won't let anything get in his way. A door? He melts it. A cashier taking too long to give one of his minions change? With a wave of his arm, the cashier is erased, leaving only a white cowering-cashier-shaped void. He's all black with white outlines and a white glow around him, and he's made of pure evil. With the confirmation that this season is the last, who could ask for a better Final Boss than Pops' evil counterpart who is voiced by Freddy Kreuger no less?