
der Größenwahn
megalomania
A pathological or severely exaggerated overestimation of one's own importance, power, or abilities. The word refers both to a psychological disorder and, more generally, to an inflated self-image.
Sein Größenwahn ließ ihn glauben, er könne allein das ganze Land retten.
His megalomania led him to believe that he could save the entire country on his own.