The best part of true horror is the feeling of helplessness thats so rare for PC's.
Keeping that in mind I had a villain once you could use.
He's a mindflayer whose completely sane with one small caveat. He believes that brains are bad for him and he must devour the raw flesh of the undead to prevent his soul from escaping. Other then that he's a perfectly reasonable guy, for a mind flayer.
When the players met him heres how i set the stage.
The first thing you notice when you open the door is the cheery glow of a warm fire and the smell of incense. The second thing you notice is a man in a hooded robe sitting behind a table set for dinner.
Most of the table is obscured beyond the doorframe but what you can see has a 2 decaying human legs cuffed to it. As you watch you can see them moving! Whoever or whatever is strapped to that table is clearly alive.
The man looks up and from the depths of his hood you hear a cheerful voice emerge "Oh its you. I've been following you for quite some time now. Do come in, I'm sure you have lots of questions and I can answer most of them. And I assure you that I will.
If they file in you say....
As you come in to the room you see the woman on the table look over and whisper out through rotten teeth "help me. My name is Sarah, he's eating me. Please by the gods help me, i have to get home."
The man then says "hush Sarah these good people dont care about that. They've come to have their questions answered. And anyway your quite dead so whats the harm? Your children would hardly like to see you in this state anyway".
Then play out the conversation with numerous breaks for the ghoul to deny she's dead and beg for help or the cowled flayer to continue eating. Make sure to mention that he has impeccable manors, uses a napkin and offers the players a little "of the juicy parts".
If you really want to evil him up you can put a room off to the side, obscured by a curtain where more voices can be heard coming from along with chains or cages rattling. Never make it clear to the PC's whether the people back there are already dead or not.
The trick to this is 2 fold.
1. Make sure the PC's know very quickly that this guy is both very powerful and dangerous and has information they desperately need. So attacking him before the conversation ends is realistically out of the question.
2. Other then eating a still begging ghoul the flayer must be the most absolutely normal, nice seeming guy you can imagine. He simply see's nothing unusual about what he's doing. He doesnt gloat, play it up or throw it in the players face too much. He's just a normal guy who happened to have guests over during dinner.