- The Deathgrippers. Part mantis, part scorpion, and part hellish demon, and a taste for the flesh of their fellow dragons, they are undoubtedly one of the more terrifying species of dragon ever featured in the series.
- Grimmel himself, in the sense that not only has he tamed the Deathgrippers by drugging them with their own venom, he is also singlehandedly responsible for wiping the Night Furies to near extinction. Think about that: the most intelligent, elusive and powerful of all dragon species was ruthlessly hunted down to oblivion by one man. One evil, murderous, bloodthirsty man. He's probably even worse than Drago.
- The scariest thing is the way that he uses clever tricks and traps, has advanced knowledge in machinery, specializes in technological inventions than brute force, and brought down a Night Fury with his gadgets. He is what Hiccup could have become if he had killed Toothless in the first film.
- His home invasion of Hiccup's house is incredibly creepy. Hiccup hears odd creaks and groans in the house and we catch a glimpse of a figure in the shadows before Hiccup ignites his sword to reveal Grimmel is right there, just casually pouring himself a drink.
- The scene ends with Grimmel's dragon bursting through the roof and setting the house on fire. It is scarily similar to Stoick's flashback from How to Train Your Dragon 2 in which Drago's armored dragons come through the roof of the conference hall and burn the Viking Chiefs alive.
- The scene where the dragons of the Hidden World attack Astrid and Hiccup, forcing Toothless to save them. It's made very clear that humans are not welcome in their secret sanctuary down below.
- And worse given Drago's Bewilderbeast was in the Hidden World! Thank goodness he didn't decide to join in the attack, especially if he recognized Hiccup and was hit with PTSD.
- Upon being alerted of Hiccup's presence, the Light Fury unleashes a volley of plasma blasts capable of smashing apart full-grown trees. As cute and pretty as she may be, she's no less deadly than Toothless at his worst.
- Gobber unleashes a swarm of Hobgobblers at Ragnar The Rock, one of the warlords. They presumably EAT HIM ALIVE.
- After an older Hiccup returns to the Hidden World, Toothless doesn't recognize him at first. The one moment of non-recognition is rather tense to behold, given Toothless might have easily killed him in defense of his young had he not recognized his old friend.
Nightmare Fuel / How to Train Your Dragon: The Hidden World