At the beginning of the game, when a blood-crazed Dracula is locked in a room by Zobek and given a family of three to sate his hunger. Terrifying.
Once you open the 'fridge' Dracula himself becomes this when you realize that the technical good guys of the setting like the Order have just about no chance of beating him, because if it wasn't enough that he's the most powerful vampire and Lord of Shadow to ever walk the earth, then The Reveal that God STILL favors him as his champion, rendering him almost completely immune to holy weapons,thus making him effectively immortal - makes things that much worse for who anyone who dares face him. Maybe in the grander scheme of things it's for the best, but it's not so great for the vast majority of ordinary people who lived under constant threat of him during the height of his power. Think about it. A nigh-unstoppable monstrosity who ravaged the land and has consumed countless innocents and with God's total blessing.
The living blood that pursues Dracula throughout his castle definitely qualifies. It turns his own monsters against him and even destroys portions of the castle in its attempts to keep him from leaving. It gets even worse when you fight Inner Dracula.
The worst part? This is Dracula's dark side manifested. This scary factor somehow manages to get even higher, when it starts whispering "I'll always be with you..." Over and over again.
The Gorgons, Eurayle, Stheno, and Medusa, all are on the side of Dracula at first, but their movements and Interface Screw make them some of the scariest monsters in the entire game.
At the end of the game, when Satan is finally summoned from Hell, the first thing he does is slowly and graphically rip a man's face off. The face is still alive when he throws it aside.
The Revelations DLC adds a whole new area's worth of nightmare fuel with the Forbidden Wing. According to Marie, Gabriel shut himself away in here after he trounced the Forgotten One — and lost his mind. Entire rooms are flooded with blood and there are phrases written in it in Latin, scattered all over the floor and walls, which are Dracula's insane ravings and curses of doom upon humanity, the Brotherhood, and Zobek. And it doesn't help that you can hear barely audible screaming and occasional laughter.
Dracula's eyes. Especially when you see them up close and he's mad or angry. The mere fact that they're red doesn't make them scary. The pupils and irises in his eyes dilate and look utterly inhuman. This, in conjunction with the fact that his eyes glow red more or less depending on how enraged the Prince of Darkness is; adds to just how fucking scary they look.
The second Acolyte may be the scariest enemy in the entire LOS trilogy. With his calm, assured demeanor and initially overwhelming power, the only thing you can do is crawl away from him like a whipped dog while he casually stalks after you, zapping your very soul with his force lightning. And what happens if he catches you? He'll open a portal and send you to the pits of Hell. His taunts during your boss fight with him certainly don't help, considering that, as a child of Satan, he knows exactly what he's talking about.
Nergal: Your suffering will make Hell itself cry! Your soul will go mad in eternal agony! Your existence will know only pain! My father eagerly awaits you in his kingdom. Don't make him wait. Perhaps my Lord will allow me to... Look after you, once he's tired of your soul.
Satan's daughter gets in on the promises of a Fate Worse Than Death for our Prince of Darkness, and anyone standing next to him.
Raisa: Ah, my dear Zobek! If only you could see what I see... I see you hanging while my brothers skin you alive! I will bathe in your blood in the presence of my father! The lost souls of the Avernus will welcome you with their cries. Bless me, father. For today I will deliver unto you the Dragon!