This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Kamen Rider Wizard
Don't be too scared. That's just the hero.
While people that die due to a monster attack (i.e Worms from Kamen Rider Kabuto spraying white liquid, and Kamen Rider Faiz's Orphenochs destroying a person's heart), Wizard no doubtly has the worst case of killing someone by Despair Event Horizon, the purple glow on a person's skin seems to stress this further. Then their body EXPLODES, and Phantoms are born.
And just like the Worms, the Phantoms can keep the deceased person's memories and take their form, effectively being able to trick anybody who knows them.
When a Phantom reveals their true identity, we're treated to a shot of their monster face superimposed over their human one, which is just plain creepy. To make it worse, said Phantom typically bares an appropriately frightening Slasher Smile while doing so.
One can see it as a sort of dark counterpart to the so-called "legend shifts" of Kaizoku Sentai Gokaiger, where we find out (or remember) what past Sentai hero the legend is.
There is almost no humor in episode 9, which makes sense since Haruto must find a way to defeat Phoenix before it's too late. The Wizardriver even sounds quite sinister and intimidating when the Flame Dragon ring is used to transform into Flame Dragon.
Haruto's nightmare where Phoenix infiltrates the hospital, murders Hiroki's parents while he watches, and Hiroki is swiftly enveloped in violet light as he turns into a Phantom.
Dragon himself is creepy. We meet him in Haruto's underworld, looking exactly like our hero except with ominously glowing red eyes and a deep voice, gloating over the scene of Haruto's dead parents. He then proceeds to do the standard 'Phantom reveal' before circling around Haruto and taunting him about the cost of the new ring's power.
Nitoh's situation most assuredly counts. Think about it, you're an archaeology student that makes the discovery of a lifetime, a belt that grants you awesome super powers, and your own monster that will fight with you...only to discover that you need to eat constantly, as well as hunt down monsters and consume them, otherwise, you're toast. Now think on what happens if you run into a monster that proves too difficult to destroy and you run out of time...or what happens when all the monsters are gone...
Beelzebub in general. His soft-spoken demeanor merely serves to highlight how monstrous he really is. What he puts the poor Victim of the Week through is just plain terrifying. Imagine having a perfectly happy life... and all of a sudden your friends start ignoring you like the plague and your formerly loving husband turns into an abusive jerk and you have no idea why. All this is made worse because Beelzebub is never seen once the entire time, making how he does it a complete mystery on top of the Nightmare Fuel that comes with his power in the first place.
The ending of #22 is pretty bone-chilling, with Phoenix generating huge ass fire wings, having them attack Haruto, and then suddenly cutting to black as we hear Haruto's scream.
Phoenix's final fate. Being trapped forever in the freaking sun, stuck in an endless cycle of death and rebirth? Brrr. Though he completely deserves it, it's still not pretty.
#24: All Gates were forcefully transformed into Phantoms. Not Nitoh's grandma...she goes to the Phantom and asks to be transformed. Why does she do this? So her grandson can EAT HER!!!
Having a monster, that looks like one of your children kill you. I can only imagine the fear of Mayu and Misa's parents...
In the same episode, we finally get to see how a Gate turns into a Phantom in full. The entire idea and implications is already scary enough, but now to see exactly just how utterly painful and horrifying the whole process is nothing short of disturbing. Not at all helped by how completely powerless the hero is the whole time.
The mere fact the Legion Phantom is the only Phantom that Wiseman considered so dangerous, so Ax-Crazy, that he imprisoned him! Keep in mind that psychos like Phoenix and Lizardman were allowed to run free...
And then there's his special power: he enters his victim's underworld to perform Mind Rape. Jesus.
Don't forget how well they characterized his human form. A simple look at his face will tell you that he is completely messed up.
His victims seem to be sleeping peacefully... then you get to Haruto, who basically goes into arrest once WizarDragon is gone. Especially grim if you or anyone you know has some form of heart condition.
#35: It is revealed that when he was a human, Gremlin/Sora was a serial killer who targeted women with black hair who were in white dresses, and he's still doing this, even as a Phantom.
Worse? He went from a regularSerial Killer to a supernaturalSerial Killer with the ability to Flash Step and who's so powerful that the police have absolutely no chance of stopping him.
The scene when The Reveal happens is particularly frightening, with the camera showing only one or two victims before it pans back to reveal three boards covered in pictures of Sora/Gremlin's victims. Haruto's reaction to this is very justified, and the set up is simply terrifying.
#36: Haruto ends up running out of mana just as he's about to activate the Drago Timer, (his power was drained by Wiseman earlier in the episode) and him being force to power down from Flame Dragon is really creepy.
#41: According to the White Wizard, if you fall into despair but hang onto hope, youHAVEto become a Wizard Rider. So here's the concept: if you get targeted by a Phantom and you fall into despair, with no Wizard Rider to use their Engage Ring, you either become a Phantom or a Rider. And that's only if you refuse the call to either option. Oi...
#45: The red crack that appears on Koyomi's hand is rather disturbing. Also seems that the Please Ring is being affected as well.
#46: Unfortunately, the crack on Koyomi's hand continues to grow and become more disturbing. Her entire right hand is cracked and she needs to wear a glove to keep it covered.
#47: Turns out that Fueki is not only the White Wizard, but Wiseman as well.
Him backstabbing Medusa was uncalled for as well, especially when we're expecting Sora to do the double crossing instead.
#48: Koyomi finds out that she's been dead all along. Even if you know you're not normal, that would still really creep you out.
Fueki saying that he needs Haruto's help to bring his daughter back, if this was earlier, Haruto might've agreed to help, but now, Haruto doesn't want anything to do with this psycho. Keep in mind the fact Fueki could place Haruto under mind control as he did to Yamamoto and Yuzuru.
#49: The second Sabbat. In order to get a sufficient amount of mana, Fueki holds a Sabbat using four wizards as the catalyst, with the ritual covering all of Tokyo. Had it not been for Beast's intervention, Fueki would have happily killed everyone in Tokyo and turned every Gate in the city into a Phantom. Japan can't catch a breakwith thesecataclysmic rituals,can they?
#50: Sora has the Philosopher's Stone. The SociopathicSerial Killer who was so evil that he became his Phantom instead of being replaced by it now has the world's most powerful magical artifact.
#51: Sora, in his new evolved form, decides to just kill everybody he can find. He needs power for the Philosopher's Stone but doesn't want to do all the work involved. If the people turn into Phantoms, they're Gates. If they stay dead then they're normal.
There's one when Haruto finally shows up to fight. He doesn't specify that he wants to save Koyomi's spirit, and this causes Rinko to think Haruto's going down the same path as Fueki. Kinda scary with the one person who trusted Haruto most thinking he's gone to the dark side.