Imhotep himself. In terms of both his appearance and personality, he feels like the character Boris Karloff was born to play.
The otherwise fantastic makeup used to bring him to life (as the page image to the right shows) deserves special mention. Every time he's onscreen, he practically dives into the Uncanny Valley. What's noteworthy about this is the fact that they don't go overboard with it. It's easy to see how people would buy that he is indeed a human (albeit a rather morose and gaunt looking one), but he nevertheless just looks off.
Whemple's assistant, Ralph Norton, going Laughing Mad upon seeing Imhotep having woken up. We learn a bit later that he was institutionalized, and died still laughing. Seriously, he was thatdamaged.
Imagine a 3700 years old thin, dry and dark mummy, supposed to be very dead and now is walking, you see it's dead dry face and it casually only took the scroll you are reading and studying.
Imhotep's murder of Whemple. The idea that he can induce a heart attack while sitting in another room in a completely separate building is prime Paranoia Fuel material.
Imhotep is not only a Paranoia Fuel treat of his powers, but also he can easily mix in the society. An extremely dangerous immortal sorcerer with knowledge in forgotten and mostly unknown mystic magic, hiding among us. He was also cursed, which is strongly implied as a source of his gained almost godlike powers. But even if he doesn't show it so much, he is also inhumanly strong, as we see bend and broken metal bars, broken by him offscreen, and he later breaks easily a resistant glass. Even if it's frightening, thank God (or gods) he's apparently only interested on resurrect his lost love.
Every single time Imhotep glares into the camera. The worst moment is the first time happens, due to coming out of nowhere in the middle of an argument just as it seemed like Muller was gaining the upper hand.
The idea that you can do nothing against Imhotep. Even a scholar as Muller says defeated that there's nothing that can damage Imhotep, an immortal sorcerer undead who want to turn Helen into an undead creature like him. It's need the direct divine intervention from Isis to stop him. At least Dracula, as powerful he was, had some weaknesses that can repel him, as the day, crosses, holy artifacts, and can be identified as a vampire more easily. Imhotep can't be harm with anything, can kill anyone with no matter how far are you from him, and the Isis amulet only protects against his powers when you touch it directly. It's only summoning an specific deity in a forgotten dead language to destroy the scroll, the only way to stop him.
