OMINOUS, PORTENTOUS, FATEFUL mean having a menacing or threatening aspect.

OMINOUS implies having a menacing, alarming character foreshadowing evil or disaster.

e.g. ominous rumblings from the volcano

PORTENTOUS suggests being frighteningly big or impressive but now seldom definitely connotes forewarning of calamity.

e.g. an eerie and portentous stillness

FATEFUL suggests being of momentous or decisive importance.

e.g. the fateful conference that led to war

source: [あんハピ♪の花小泉杏たちの前を黒猫が通過するGIF画像｜無料GIF画像検索 GIFMAGAZINE 940333]

giving the impression that something bad or unpleasant is going to happen; threatening; inauspicious:

不吉な, 胸騒ぎのする

e.g. there were ominous dark clouds gathering overhead.

ominousness | ˈämənəsnəs | noun

late 16th century: from Latin ominosus, from omen, omin- ‘omen’.