Which word describes something of momentous or ominous significance?

The term "portentous" specifically refers to something that carries a significant, often negative, meaning or implication, suggesting an important or grave outcome. It embodies a sense of foreboding or warning about future events, which aligns with the description of something of momentous or ominous significance. This word captures the essence of a situation or event that is substantial in terms of potential impact or consequence, often evoking concern or anxiety.

In contrast, "prosperous" pertains to success or thriving conditions, generally in relation to financial or overall well-being. "Palliative" refers to something that alleviates or mitigates pain or suffering without addressing the underlying cause, and "protective" relates to measures taken to guard against harm or risk. While these words have their specific meanings and contexts, they do not convey the sense of foreboding or significant consequence that "portentous" does.