A delusion is a belief in a possible reality in which one is absolutely convinced that their perspective is the ultimate truth. Delusions reject the possibility of fantasy in an altogether absurd manner.
You see there is a vast difference between believing that you are, in fact, napoleon and fantasizing about what it might have been if you were Napoleon.
A delusion is like a felon of the mind. It has broken lose from the confines of quixotic musings of its' owner and wreaks havoc on the diluted perception by which it manifests itself.
In fantasy, one recognizes that it is an alternate reality that satisfies our deepest desires and expectations without actually projecting itself as an attainable alternate reality. One indulges in a fantasy but has the means to acknowledge their formal and objective reality.
Delusions seep forth from the desperate need to realize a particularly forceful fantasy. When reality begins to weigh heavy on paper hearts, they are forced to rely on the beauty of their delusions to paint the world as something more.
But the danger does not lie in believing in the delusion in itself; the danger lies in its destruction. For a fantasy can end without serious consequences to it's creator; but the death of a delusion breeds insanity.
For if what you believed to be true, suddenly collapsed onto itself, you are left with the brutality of what is real, without the comfort of your alternative. You are left to look out upon the world as it is, not as it should have been.
And so while fantasy breeds little harm, so long as one recognizes it as such, delusions will not allow themselves to be recognizes as anything more than reality serving subjective beliefs.
It is terribly difficult to remove yourself from the tenacity of delusions. They are a false horizon. Yet they breed hope. Without delusions to assuage the truth of reality, we are left alone in a world that is indifferent to our existence.