MYTH: Narcissists Are Highly Intelligent

Narcissist Myth: "They are extremely intelligent."

Reality: High IQ does not a Narcissist make, and in fact it more often than not can bolster an individual's understanding of ethical and courteous treatment of all people.
There are Narcissists who have high IQs. However, having even a "genius level IQ" does not mean a person possesses Global Intelligence. In other words, you can be a brilliant mathematician and be dumb as a rock in other things. Global Intelligence means you have high function AND interconnectedness between the different parts of the brain. You might still only be a "genius" in one or two areas, but those are usually because that's where your passions lie, where your heart is in.
Global Intelligence means you can integrate all kinds of things, including how to treat other people and WHY civil and courteous behavior is absolutely essential to a healthy society, healthy family, healthy individual, healthy life. Ethics and respectful social courtesy and personal responsibility are quite mathematical, very logical, and very important; there is nothing "trivial" about them, or unnecessary. Only a person who is not very good at logic would treat others with the disdain and disrespect that Narcissists hand out on a regular basis.
A Narcissist with a high IQ can not have Global Intelligence, so their "genius" can only, ONLY, lay in their field of focus, and without ethics, their "genius" is often muddied or even obliterated by a lack of understanding of the actual implications and effects of what they are trying to accomplish. He might be the best violin-maker that ever lived, but he is going to lose paying customers, drive away business, and lose any assistant talented enough to work with him due to his behavior. There is a good chance also that instead of learning from anyone else who is a talented violin-maker and enjoying the camaraderie of a like-mind, he will instead try to undermine them. Also, the high IQ Narcissist is applying (wasting) much of his brain power to garnering power and recognition for himself instead of putting his focus on his work, and so the work always suffers. He usually achieves mediocrity, which serves to increase his Narcissistic rage. (Antonio Salieri in Amadeus).
There are plenty of Narcissists with lower IQs as well. Where a person applies their focus is where they "get good at" stuff. So if a person with a lower IQ has focused on social manipulation since they were in their youth, they will have achieved expertise by the time they reach adulthood. As one Narcissist said to me several years ago, "Genius is as genius does."
(I'm sure he made that up on his own.)