portentous

Borrowed from Latin portentōsus, from portentus; equivalent to portent +‎ -ous.

portentous (comparative more portentous, superlative most portentous)

1. Of momentous or ominous significance.

   Synonyms: ominous, momentous

   • 1931, H. P. Lovecraft, The Whisperer in Darkness, chapter 2:

     It is no longer in my possession, but my memory holds almost every word of its portentous message; and again I affirm my confidence in the sanity of the man who wrote it.

   • 2019 March 15, Rachel Aroesti, “The Cinematic Orchestra: To Believe review – soundscape originators' accomplished return”, in The Guardian‎:

     The main problem with To Believe, however, is that by combining two of their previous styles, the Cinematic Orchestra have landed on a sound that has become ubiquitous in the years they’ve been away – portentous, restrained and impeccably tasteful electronica.

2. Ominously prophetic.

   Synonyms: foreshadowing, predictive, premonitory, prognosticatory

3. Puffed up with vanity.

• portent