Portent(n.) That which portends, or foretoken; esp., that which portends evil; a sign of coming calamity; an omen; a sign.
Portentive(a.) Presaging; foreshadowing.
Portentous(a.) Of the nature of a portent; containing portents; foreshadowing, esp. foreshadowing ill; ominous.
Portentous(a.) Hence: Monstrous; prodigious; wonderful; dreadful; as, a beast of portentous size.
Words within portention