Comet of Doom

"When beggars die there are no comets seen; The heavens themselves blaze forth the death of princes."

—Calpurnia, Julius Caesar by William Shakespeare

Before people had telescopes, comets were frightening objects of awe and wonder that seemed to appear out of nowhere, blazed brightly in the sky, then vanished as quickly as they came. For thousands (and perhaps tens of thousands) of years, they were seen as bad omens, and pronounced the deaths of kings or the coming of a horrible disaster. The "falling stars" mentioned in the tale of Gilgamesh was possibly a reference to comets or meteor showers.

In 1705, astronomer Edmund Halley noticed that the comets of 1531, 1607, and 1682 all had the same orbit and period. Suspecting that the three were actually the same comet, Halley predicted that it would appear again in 1758. It did, and Halley's Comet not only acquired a name, but put an end to the thought that a comet was some supernatural envoy of doom.

Be that as it may, comets remain cool, and they are still used in media as the first, last and only suitable omen for truly world-shaking events. Such as said comet coming straight at you...

Subtrope of Portent of Doom.

Examples of Comet of Doom include:

Anime and Manga

In Space Carrier Blue Noah, the aliens rain down surveillance cameras disguised as meteors onto planet Earth, to scope out whether the planet is worth invading.

Film

Lifeforce. The arrival of Halley's Comet foretells doom for London, as it contains an alien spaceship that carried space vampires. Discussed, as the characters mention that the appearance of Halley's Comet has been considered a warning of disaster for centuries, possibly because seeing it meant the alien ship within was near enough to allow the vampires to reach Earth and feed.
Night of the Comet. A comet's trail sweeps across the Earth, reducing almost the entire human race to reddish dust.
The romantic comedy Wimbledon actually has this: Paul Bettany's character seems to take on l33t tennis skills only while a comet is in the sky. And yes, he wins Wimbledon, because everyone knows it will take the intervention of God for an Englishman to ever win that tournament again.
In The Brainiac, a comet carries a magician who escaped execution by transporting himself there and who then returns to earth 300 years later as a monster with forked tongue which he uses to suck peoples brains out.

Literature

In Larry Niven's novel Lucifer's Hammer, pieces of a comet slam into the Earth and destroy civilization.
Ditto the book Comet Dis'aster, a book that incidentally has a lot of terror spread about it.
As the page quote reveals, a comet appears in the sky after the assassination of Julius Caesar, in Shakespeare's play of the same name.
In A Clash of Kings, a comet appears in the sky over Westeros, and is taken as any number of omens, both good and ill.
In Tad Williams' Memory, Sorrow, and Thorn trilogy, the Conqueror Star is a highly visible comet whose coming is said to portend the fall of empires. Given the limited amount of information presented in the story itself, it's hard to say whether it actually has supernatural powers, but its periodicity is certainly very odd.
The Red Star in Dragonriders of Pern is a captured planet in an elongated (comet-like) orbit. When you can see it in the sky, it means that it's nearby... and has dragged a bunch of frozen organisms from the Oort Cloud to the inner solar system with it. These organisms, upon entering Pern's atmosphere, become the Threads. They eat any organic material they can. Crops, wood, grass, fungi... people...
In one Animorphs book the characters have traveled to the time of the dinosaurs and see a comet in the sky. Cassie mentions that humans used to believe that comets were a bad omen, and Ax says that Andalites had a similar superstition. Of course, said comet turns out to be the comet that killed the dinosaurs.
A comet pops up during the siege of a town in Colas Breugnon, and is taken to be a bad omen, though no one can agree on what it is an omen of or to whom it is bad.
In the Black Company books, a Comet shows up twice in the story and once in the backstory, each time to portend the White Rose coming to battle the forces of the Dominator.

Live-Action TV

In Babylon 5, the Brakiri solar system contains only one comet with a period of 200 years. The comet is considered a death omen (and the focus of the season five episode "Day of the Dead"). As a result, the Brakiri don't even like comets to be mentioned.
In the television series Beauty and The Beast, a comet was blazing in the sky the night billionaire Elliot Burch is murdered by operatives of his rival, Gabriel. Father marks the occasion by speaking the page quote.
Doctor Who: In Silver Nemesis there is a comet with a period of 25 years that, according to the Doctor, really does bring misfortune: it's actually an alien superweapon that somehow (ahem) wound up in a solar orbit. He cites the the two World Wars and the assassination of JFK as the results of the last three times it came near Earth.

Mythology & Religion

In a rare example of a comet being a good omen, some scholars believe that the Star of Bethlehem was one.

Tabletop Games

In the Dungeons & Dragons adventure module X2 Castle Amber, the appearance of a blazing red comet over Averoigne causes an NPC to become a deadly monster.
- "King-Killer Star" of Forgotten Realms. It did cause damage, and turned out to be just a random comet with right period chosen as the trigger condition for a magical device -- until the latter was "hacked".
In the Shadowrun supplements The Year of the Comet and Wake of the Comet, the passing of Halley's Comet in 2061 greatly increased the magic level on Earth, which caused SURGE (Sudden Recessive Gene Expression) mutations in (meta)humans, animals and plants, the re-appearance of the dragon Ghostwalker and natural disasters in the Asia that brought down the Japanese Empire.
The presence of a twin-tailed comet is often seen as an omen to the people of The Empire, due to the legend that their founder Sigmar's birth was heralded by it's appearance. This is not always seen as a good omen, however, especially to the city of Mordhiem, which was hit by one.
In the Alliances expansion for Magic: The Gathering, General Varchild saw a comet which she believed meant that she was to conquer the barbarian tribes as a part of a manifest destiny for her people. So she took her army and began the slaughter.

Video Games

In Ultima V, each of the Shadowlords has their own comet; if you have a telescope you can determine which city they're in by looking for where the comets are.
Final Fantasy VII. Though it's technically a meteor, and not so much an omen of The End of the World as We Know It as the cause of it.
- Final Fantasy V also has prophetic falling heavenly bodies, but they are used to help save the world instead of destroy it.
Super Mario Galaxy has the Prankster comets.
A random event in Europa Universalis involves a comet or meteor appearing and the superstitious peasants being concerned enough that your country loses stability.
Illusion of Gaia's entire plot is influenced by a comet that returns every few hundred years. Each visit affects all life on Earth in unexpected ways, although the end results thus far (that tend to linger even until the next visitation) are usually the same (mutations, destroyed civilizations, famine, etc.).
In the Call of Cthulhu (tabletop game) PC adventure game Shadow of the Comet, the passing of Halley's Comet coincides with some truly creepy stuff going down.
A comet herald for the arrival of Balor in Myth.
Gadget: Past as Future. The player has to gather gizmos to build a spaceship before a comet hits some Steampunk-ish, Nineteen Eighty-Four-esque eastern European nation.
In Shadow the Hedgehog, there is the Black Comet. Not an actual comet, however, but a meteoroid that serves as the vessel of Black Doom and his army. Black Doom uses the Chaos Emeralds to teleport it into the atmosphere and start sucking energy out of the planet. Fortunately, it turns out that the space colony A.R.K.'s Eclipse Cannon was created to blow the Black Comet to smithereens. After Shadow teleports it back out into space.
In Baten Kaitos Origins, Guillo's strongest special attack, Aphelion Dustwake, showers enemies with the ice trail of a comet. It's quite overpowered.

Western Animation

Happens at least one time in The Fairly OddParents. Of course, Timmy has wished for there to be no noise, and he cannot wish away the comet. Until he figures out that he can use charades (conveniently played in the early part of the episode) to wish for sound.
This happened at least once in Xiaolin Showdown. A comet accompanies a mysterious event that could quite easily cause The End of the World as We Know It.
Sozin's Comet from Avatar: The Last Airbender: Its arrival meant a major power boost for the already powerful Firebenders and near certain doom for the Earth Kingdom (Which Fire Lord Ozai planned to literally burn to the ground using the comet boost)
- In the backstory, the arrival of Sozin's Comet a hundred years ago and the power boost it provided for Fire Lord Sozin and his Firebenders heralded the near-extinction of the Airbenders.
Comet in Moominlands main plot.

Real Life

A comet was seen in the sky shortly after the death of Julius Caesar. Rather than being seen as a bad omen, the comet was believed to be proof that Julius Caesar had ascended to godhood after his murder.

"On the very days of my Games a comet was visible for seven days in the northern part of the sky. It was rising about an hour before sunset, and was a bright star visible from all lands." -- Caesar Augustus

Tacitus mentions two comet sightings during the reign of the Emperor Nero. Nero was apparently so concerned about each of these two comet sightings (in the years 60 and 66 AD, respectively), thinking that they were omens foretelling the sudden end of his reign, that he had most of the Roman nobility massacred as a means to forstall assassination.
- Just as an interesting aside, the comet in 66 AD was Halley's Comet.
Months prior to the Battle of Hastings in 1066, Halley's Comet appeared in the sky. The Bayeax Tapestry records not only the appearance of the comet over the battle, but King Harold II of England being informed of the appearance of this ill-omen.
In 1996, amateur astronomer Chuck Shramek announced that he had found a "Saturn-like object" following the Hale-Bopp comet (closer examination, however, proved that no such object existed). Unfortunately, UFO enthusiasts everywhere latched on to the supposed "fact" that there was an alien spacecraft following the comet. Among the people who got sucked into this fantasy were the members of the Heaven's Gate cult, who chose the appearance of the comet as a signal for their mass suicide. They claimed they were "leaving their earthly bodies behind to travel to the spaceship" that was supposedly following the comet.
Mark Twain was born on November 30, 1835, 20 days after the passing of Halley's Comet on November 10. In 1909, he had stated, "I came in with Halley's Comet in 1835. It is coming again next year, and I expect to go out with it." Twain passed away on April 21, 1910, the day after Halley's Comet passed again.
Older Than Dirt: Mentioned on ancient Chinese oracle bones from the late Shang dynasty. A comet was also among the omens thought to have foreboded the fall of the Shang dynasty by the victory of King Wu of Zhou over King Zhou of Shang, c. 1050 BCE.