The principal characteristic of narcissism, whether one wishes to use a Freudian understanding or one drawn from popular psychology, is that of a disconnect between one's emotional life as well as one's intellectual and spiritual life. Or to use one of Nietzsche's telling phrases, it is a "denial of reality." Narcissism considered as a problem requires consideration of "reality." In essence, narcissism leads to the creation of an alternate reality: one in which one's dreams, desires, hopes and plans, which would otherwise go unfulfilled, may find fulfillment.
Consequently, the reality principle, to use Freud's phrase, is simply that fulfillment of those dearest dreams, hopes, and plans have no guaranteed fulfillment. Further, reality always means the Unexpected. The dream of guaranteed fulfillment motivates the creation of narcissistic reality. Reality has sharp, rough edges. Metaphorically, and not so metaphorically speaking, reality is the sharp table corner a small child happily at play suddenly discovers by running into it while at play.
Power considered in this context means the capacity to overcome the shocks to which the flesh is heir. A given quantum of power allows unexpected events of a certain intensity to go unnoticed. Power makes the sharp, rough edges of reality smooth. In fact, some of those unexpected corners come to be perceived as smoothness itself. Consequently, it is intrinsic to power that it obscures perception and makes denial of reality into something more than a simple recipe for disaster.
If this last paragraph should seem especially opaque, consider the example of any true believer and his ideology. A Christian, for example. It is enough to contemplate sitting in judgment of angels, for the effects, causes, and consequences in this life to become of little consequence. The fantasy of being at some future date on the winning side is enough to incite feelings of power and consequently, of the capacity to ignore the trials and tribulations of this life, even though there is no guarantee of no other.
The underpinning of the lust for power is fear of frustration and hurt in this life. Hence the need for the Christian God to be both all powerful and all good.