Betrayal. A trusted person gone bad--so bad that s/he is willing to betray her/his closest friends.
It's something many books should have--and there's a certain way to do it. Hopefully, in this post, I can show you a few of those ways.
Foreshadowing is a useful thing--especially when betrayal is involved. If your story has more than one POV, this can be a useful thing--and foreshadowing can happen without multiple POVs as well. Here is two of the main points of foreshadowing.
1. Protagonist foreshadowing. This is done when you hint at something suspicious, using your Protagonist. In The Door Within, Aiden sees his 'betrayer' sneak out of bed into the night. He assumes the Glimpse had gone to relieve himself--but later, you find out that he had
betrayed them. This kind of foreshadowing makes you say, "Why didn't I see this coming?" Its main point is creating shock--a good thing for an epic story.
2. Multiple POV foreshadowing. This is where you go into an 'enemy' POV and basically tell the reader that someone is going to betray the Protagonist. I'm sure you can think of a couple stories that have this example--but I can't think of any as of right now. M-POV foreshadowing's main purpose is 'dread' and suspense. Dread is a sense of impending danger, and, of course, you know what suspense is.
2) I Didn't See This Coming
Like Protagonist POV foreshadowing, this is the 'I didn't see this coming' part of betrayal. There are basically three kinds of IDSTC;
1. Foreshadowing, like discussed in the first point of the foreshadowing part of this post.
2. Making you think someone else will betray them. I have read this in many books--and that has a reason; it is a good way to shock someone. Say you have the Protagonist and two companions. The Protagonist is warned, one way or another, that one of his/her companions is going to betray him/her. The Protagonist immediately suspects one person...but after the betrayal takes place, s/he finds out it was the other companion that was the traitor. Misleading your reader into thinking someone is a traitor--who actually isn't the traitor--is a great way to make a betrayal all the more shocking.
3. Making a very close friend betray the Protagonist. This works wonderfully because it is unexpected. In fact (I really should have said this earlier), the greatest tool for betrayal is to make it unexpected. But since close friends--and even family--are usually held as 'good' guys, it makes it shocking. Judas was an extremely close friend--one of the Twelve--to Jesus, yet he betrayed Him.
Which is the best way to do this? Well, I've said this many times before on various subjects, but I'm going to say it again; do both.
Make the person a close friend--and mislead the reader as to the betrayer at the same time. Make the Protagonist foreshadow the betrayal--and (if you can) make an enemy tell of the betrayal upcoming. Do this, and your betrayal will be even more shocking and terrible--the goal of a good betrayal.