Damien. The concept of the Antichrist being a child who is fully aware of the terror that he is directly and indirectly causing is freaky. What could make that worse? The fact that he acts so innocent and adorable most of the time.
How about Patrick Troughton as the priest trying to find refuge at the church only to get skewered on a lightning rod. Nightmare Fuel for running from... well, Satan. It's also Paranoia Fuel for the reason that even churches and holy grounds can't keep you safe.
The evil Rottweilers.
What is it with the nannies in this film? One's hanging herself to death during a party, admittedly it was during the devil's son party, so it was probably the perfect gift. Then we get a new one with eyes so evil no parent in their right mind would allow her near the house never mind their child.
Mrs. Baylock: Have no fear little one. I am here to protect thee.
It's creepy enough looking at an ancient painting showing the face of The Antichrist, it would be even worse when he looks just like your adopted son.
The ironing scene in the third movie. The fact that we don't actually see Dean's child getting ironed to death really doesn't help.
Thoughout the first three films, it becomes clear Damien's followers are everywhere and could be anyone.
The most terrifying moment is at the end of the first film where Robert Thorne goes into the church dragging Damien inside in order to kill him by stabbing him with knives. All the while Damien is screaming "No, Daddy, no!". Yes, it's The Antichrist, but still...
Keith Jennings getting his head sheared off by a plate-glass window. The original version is arguably more frightening due to the film makers pulling it off convincingly without the aid of special effects.
Then there's the graveyard scene, when Robert Thorne digs up the grave where Damien's real mother lies, only to find the skeleton of a jackal resting in there. He then digs up the child's grave, with the logic that if it was the skeleton of a jackal cub then his real son was still alive. But upon finding a human baby skeleton inside, the sorrow on his face is painful to watch.
Maybe so, but the build up to the murder was terrifying. For one thing, we have no idea Baylock is even in the hospital so when she opens the door we only have no idea it's her. But when Kate turns around and sees the sheer evil in her eyes it's heart stopping. It's so terrifying(to her) she can't even scream.
The scene with the troop of baboons going crazy when they realize who Damien really is.
Damien killing Mark via a brain hemorrhage in the second film when the latter refuses to join Damien's quest for world domination.
Most scenes involving Mrs. Baylock. Her ability to seem so collected and proper for one scene, than outright demonic in another. The best example is the climax where she sneaks up on Mr. Thorn and starts biting and clawing at him and she just won't go down. It takes two knives in the neck just to stop her. Or a car if you're going by the remake.
From the fourth movie, when this woman was bitten by snakes and died under the shadow of the upside-down cross? The music did not help at all.
Also from the fourth movie, when that detective was hearing people singing Christmas carols as he saw the little Christmas statues of the baby Jesus, this happened: he gets a vision of the baby Jesus statue turning demonic and the carolers turning into creepy Satanists, chanting a spooky chant in Latin and flipping a cross upside down. .
The mother's nightmares in the 2006 remake are examples of this as well. The first having her in a tub seeing Damian wearing a wooden mask, then cut to her face, then back to Damian making an fast inhuman scream while holding the rope the first nanny hanged herself with. The second was her brushing her teeth when she hears something. There's nothing but when she closes the bathroom mirror, there's a red-cloaked... thing wearing an animal skull over its face.
The remake shows how preposterously easy it is to kill a hospital patient.
In the original film, the nanny's eyes when she kills Catherine.
*Smiles* "Damien! Damien, look at me! It's all for you!" *Jumps*
Joan Hart's death, with huge heapings of Eye Scream.
The fact that Damien has all the help in the world, both supernatural and from human followers, while the good guys are basically left to try and take down the Anti Christ by themselves with no assistance whatsoever from any divine forces, with a predictable result. You'd think God would be a little more keen on keeping the Anti Christ unborn, but nope, humanity is left to fend for itself as usual.
Justified when you see the end of the third movie, where the Second Coming happens despite all Damian's scheming. As in the Indiana Jones movies, it turns out God had it handled all along. You'd think priests would have enough sense to trust the omniscient, omnipotent entity they worship to look after itself, and the prophecy to play itself out exactly as predicted.
Damiens omnious last words in the third movie. Since Omen IV can be pretty much considered non canon, its left to the viewers imagination what he meant.