Question: I've always had difficulty making the villain truly clever, intelligent, and crafty. I know this depends on aspects such as history, mentality, overall personality, but is there a secret to making them seem especially dark and cunning? Not just in their actions, but in the way they speak. That's another problem, I'm not sure how to write villain dialogue and make them sound intimidating.
One ingredient to villainy which you might consider is the concept of "ulterior motive."
Historically, what makes villains so untrustworthy is that they do not state their real motives up front. They are dishonest or not open about their real plans, such that people cannot trust them. This also makes them outsiders to their community. (This dates back to the times when people knew the members of their community very well and people relied on their friends, family, and neighbours. Trust mattered much more perhaps than it does today.)
Something else about villains is that they pursue their personal ambitions regardless of other people. In this, they resemble sociopaths who lack empathy for others. They try to manipulate situations and people.
One easy way to make a villain villainous is to hide his true motives and intentions. In every scene or conversation, give him a hidden agenda that has to do with gaining some advantage. The main character's challenge is to figure out what the villain's real agenda is.
Only when his/her plans are advanced to the stage where success seems inevitable will the villain feel confident enough to reveal them (which is why villains tend to monologue at the crisis). That's also when they want admiration for what they have achieved to feed their ego.
Alternately, some villains will reveal their plans only after being defeated.
If you're not a manipulative, power-hungry person, it can be challenging to put yourself into the mind of such a character, but with a little imagination you can do it.