A delusion (common in paranoia) that you are much greater and more powerful and influential than you really are WordNet 3.0 & Open English WordNet

Grandiose delusions, also known as delusions of grandeur or expansive delusions, are a subtype of delusion that occur in patients with a wide range of psychiatric disorders, including two-thirds of patients in a manic state of bipolar disorder, half of those with schizophrenia, patients with the grandiose subtype of delusional disorder, frequently as a comorbid condition in narcissistic personality disorder, and a substantial portion of those with substance abuse disorders. Wikipedia