This darkness, whatever it is, needs to correlate with the horror aspect in a way that the reader is only vaguely aware of the connection while they are reading but highly aware of the connection after they are through.
The reading experience for them should be like any other delicious consumption, simple, riveting, easy to continue and difficult to cease. The audience should feel super connected with your character as they deal with both a straight up Scary (the horror aspect) and the horrifying discoveries they make about themselves as a human. The Shining isn't just about an evil hotel, of course; it also showcases the gritty and painfully real emotions tied into alcoholism, anger management, and how family life is affected as a result of the two.
To avoid an empty horror book with no heart and cheap scares, it is so important that the terrible things in your novel are not just happening around your main character, but with them. They, and in turn, the reader, should feel an uncomfortably intimate connection with this Big Scary due to the horror that is currently happening within themselves. It's this perfectly balanced dance between the contemporary-horror and the horror-horror that will produce a truly knockout final book that people remember.
And, of course, something that will scare the holy hell out of them.