IPA: //ˈɡæstliər//
KK: /ˈɡæstliər/
More frightening or shocking than something else; extremely unpleasant or horrible.
The movie was even ghastlier than the last one I saw.
Ghastlier → It is formed from "ghastly" (from Old English "gastlic", meaning ghostly or terrifying) and the suffix "-er" (meaning more). The word "ghastlier" means more ghostly or more terrifying than something else.
Think of something that is 'more ghostly' or 'more terrifying' — 'ghastly' relates to ghosts, and '-er' indicates a comparison.