Messenger of Doom

This trope is normally associated with characters whose appearance is to bring forth a message of impending danger to a group of people to whom the message is being delivered. The character in question could be sent by someone as a herald, either for purposes of good or evil, or that could be their own life's mission, having witnessed something terrible or having known a terrible secret that he or she must reveal for the intended recipient's future survival.

Comics

 * Harbinger and, to some extent, Pariah in DC Comics' Crisis on Infinite Earths.

Literature

 * Tuor, the son of Huor, first cousin of Túrin from The Silmarillion. Tuor was chosen by the Vala Ulmo to fulfill his prophecy of a messenger to warn Turgon of impending doom of Gondolin. Tuor's warning was ignored and Gondolin fell, but Tuor escaped and saved many survivors.

Religion

 * Prophets in the Old Testament were considered this to the people they were sent to, though in certain cases they were also sent to preach a message of hope.

Tabletop Games

 * The Dark Prophet from Mutant Chronicles, a badass messenger of doom, who is named... Billy.

Western Animation

 * Grandpa Smurf in the four-part episode "Smurfquest" from The Smurfs returns to the village after a 500-year quest around the world to warn the other Smurfs of the impending danger of the Long Life Force Stone losing its power.