The opening to the film, once people fail to pull the sword from the stone; it cuts to a star then a dark forest with the voiceover turning very ominous: "This was a dark age, without law and without order. Men lived in fear of each other, for the strong preyed upon the weak." An understatement, if you have any historical knowledge at all. The Dung Ages is mostly a myth, but it was still an insanely violent and dangerous time.
Despite the wolf being portrayed in a comical manner, it can be quite scary seeing how close he keeps coming to catching Wart. Then he re-enters the story when Merlin and Wart are shapechanged into squirrels and he's a much more credible threat.
Archimedes repeatedly trying to save Arthur from the pike, while Arthur cries desperately for help.
Merlin stating that even the underwater realm has its own monsters, and even sings a short melody about how there are always predators hiding in the shadows.
The hawk suddenly appearing and almost catching Wart.
Mim's creepily calm voice when she reveals to Bird!Arthur that she's going to kill him. And she shape shifts into a cat to make it a more fun game. And when he pecks her nose in defiance, she decides to stop playing and threatens to wring his neck.
A bit of Fridge Horror there, too. Archimedes is noticeably terrified of her, suggesting that she has a nasty reputation. The way she tells Arthur that she's going to kill him is that of someone making a casual (if negative) observation. Her only reason to kill him is the fact that Merlin considers him good. This combination of facts implies that Mim has killed people before. And felt absolutely zero remorse for it too!
Madam Mim hiding her head behind her hair, then revealing that she changed it into an ugly pig face.
Mim disappearing and then her eyes and jaws appearing behind Merlin, who only just sees it in time.
Alternatively, the Impossible Hourglass Figure form Mim takes can come off as creepy because how different it is from the other character animations, especially her face and eyes.
