One of the biggest challenges in writing fantasy is creating a real villain. More often than 
not, they’re one dimensional, power hungry cartoons who don’t actually seem to have a good motive for their evil ambitions. Just like the protagonist, the antagonist requires a level of depth that constructs and develops them like a real person. But since so many genre writers fall into this trap, it’s easier to create a villainous character when following these four simple rules:
Rule 1- Give Your Villain A Life: Don’t plop the villain into the story for convenience. Give them a back-story, a life with ups and downs that led them to the point where the reader meets them.
Rule 2- Give Your Villain a Motive: As I said before, many stories contain rambling villains who chase the ball but don’t know why. Give them something to work for. A goal they want to obtain.
Rule 3- Let Your Villain Grow: Just as the protagonist develops, grows and changes with the conflict, so should the villain. Even if it’s for the worst.
Rule 4- Treat Your Villain With Respect: If you’ve ever seen a Bond film, the villain will reveal their evil plot, or ramble on about how they will beat the hero. Don’t do this. Ever. If you’ve built your villain with the aforementioned rules, they will act first and ramble later.
Now, Rule 5 is more of a writer specific rule. Something you as the author are required to understand but the reader doesn’t necessarily need to know. Rule 5 is See Your Villain as They see Themselves: People with ill will towards others do not see themselves as evil, vile or villainous. Try to understand and comprehend your villain as the hero in their own right. Believe it or not, good and evil is not all black and white. There’s quite a grey area in the mix.
With these rules in mind, you’ll be able to create a dastardly villain that is not only believable enough to jump off the page, but so multidimensional that the reader may at one point sympathize with them or perhaps even root for your villain to succeed.
There And Draft Again 
I love #5 and that’s so important to remember. In fact, some bad guys see themselves as the good guy.
It’s sad, but I didn’t realize until recently why mine thought he was the good guy. It’s amazing how many facets of the story become deeper with just that one change.
I actually prefer a story where the bad guy is bad, but you can kind of see where he’s coming from. Sucks me right in. (But if s/he is too sympathetic, you’ve probably lost me. We have a hard job, don’t we?)
I just finished Joe Abercrombe’s series — his character Logen got to me. The guy is somewhat nonredeemable, but the way he wrote him . . . dang it.
I hate bad guys that monologue instead of ‘just getting on with it’ – even if he is over-confident and allegedly unstoppable.