Example : A baleful look.
Forbidding thunderclouds.
His tone became menacing.
Ominous rumblings of discontent.
Sinister storm clouds.
A sinister smile.
His threatening behavior.
Ugly black clouds.
The situation became ugly.

Synonyms : baleful, forbidding, menacing, minacious, minatory, ominous, threatening

Example : Black deeds.
A black lie.
His black heart has concocted yet another black deed.
Darth Vader of the dark side.
A dark purpose.
Dark undercurrents of ethnic hostility.
The scheme of some sinister intelligence bent on punishing him.

Synonyms : black, dark

Example : Bar sinister.