Synonyms:
portentous, prodigious
Meaning: of momentous or ominous significance; "such a portentous...monster raised all my curiosity"- Herman Melville; "a prodigious vision"
There were circumstances, moreover, which coupled with late events, gave an unearthly and portentous character to the mania of the rider, and to the capabilities of the steed.
The impressive phenomena which characterize it, the prodigious noise, the awful flash, the portentous gloom, the blast, the rain, have left a profound impression on the myths of every land.