A portent is a sign or omen. Storm clouds may gather portentously on the horizon.
Something which is like, or serves as, a portent is portentous. Anyone coming to this blog for the first time might consider the opening line of this post a portentous warning, especially if they’re not fond of really terrible puns.
Another meaning of portentous is pompously solemn. Don’t think anyone can accuse me of that.