It is a person who suffers from a serious personality disorder that consists of having delusions of grandeur. This person does not mind lying, exaggerating or manipulating in order to obtain their goals.
Megalomaniac,-na adj. s. m. and f. 1 applies to the person who suffers from a mental disorder which takes you to believe more important than it is. 2 Applies to the person who has an excessive desire for greatness.