siniestro2

² = ominous, sinister, dark [darker -comp., darkest -sup.], spooky [spookier -comp., spookiest -sup.], spine-tingling, portentous.

Ex. At first blush, nothing seemed particularly ominous about the formation of the ad hoc committee.

Ex. The selectman received this explanation in silence, but he fastened on the librarian a glance full of sinister meaning.

Ex. The novel is disturbingly dark, violent, and filled with iconoclasm, despair, and paranoia = La novela es inquietantmente siniestra y violenta y está llena de iconoclasía, desesperación y paranoia.

Ex. Records are even being sold with terrifying sounds designed to create a 'spooky' atmosphere at home.

Ex. This is a spine-tingling collection of real haunted houses and spooky ghost stories.

Ex. Before me stretched the portentous menacing road of a new decade.

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* de un modo siniestro = spookily.