Often the terms "horror" and "terror" are used interchangeably, but sometimes they are used and interpreted incorrectly. There are subtle difference in shades of meaning of the 2 words. The difference between terror and horror is that terror occurs in anticipation of the horrifying experience, while horror occurs after. Very simply, "horror" is the aftermath of "terror."
The way to properly understand the difference between horror and terror and when it comes to utilizing these terms is all about intent. Terror is the feeling of anticipating something horrible. Note that it comes before the bad experience. Horror is the feeling that one can get from a scary situation. When one is frightened by a scenario. It can be a ghost, a killer, or just a general feeling, mood, or ambience. Terror, on the other hand, has a specific person or entity behind it. Terror is the act, which is inflicted on people. For instance, a serial killer has the express purpose of scaring or hurting others. To terrorize is to specifically go after people, with the purpose of fright, intimidation, or harm. So one is a general feeling, and the other is an intended malicious act on a person, or more likely, a group of people.
You should never say it will be horrible as that thing you are describing is yet to occur. So it will be terrifying is the correct usage. Horror is the feeling of remembering that experience. Both are feelings of revulsion but one comes before and one comes after.
So, here's your English lesson that nobody asked for but got anyway.