The adjective form of megalomaniacal is megalomaniac
Mm
Definitions
adjective megalomaniacal Also, megalomaniacal [meg-uh-loh-muh-nahy-uh-kuh l] /ˌmɛg ə loʊ məˈnaɪ ə kəl/ (Show IPA), megalomanic [meg-uh-loh-man-ik] /ˌmɛg ə loʊˈmæn ɪk/ (Show IPA). of, relating to, or suggesting megalomania or a person who is afflicted with it. 1