Sentence:  Think of wandering amid sepulchral ruins, of stumbling over the bones of the dead, of encountering what I cannot describe,—the horror of being among those who are neither the living or the dead;—those dark and shadowless things that sport themselves with the reliques of the dead, and feast and love amid corruption,—ghastly, mocking, and <WSD>terrific</WSD>. 
Meaning of the word marked with <WSD> tags:  Terrifying; causing terror.