Some ancient cultures referred to them as "the Menace of the Universe" and "the Harbinger of Doom." Comets have almost universally been viewed by the ancients as messengers or omens carrying bad news from the gods.
In the midst of our busy lives it's difficult to keep in mind that we reside on a ball that's corkscrewing through space around a ball of gas called our sun, which is also sweeping through our galaxy.
The ancients didn't have distractions like TVs and the Internet. They looked to the sky for clues and guidance for life on Earth, and comets brought mythology of angry gods and instilled fear in them.
Strangely, in more modern times, comets have been associated with actual dark events.
According to NASA: Comets' influence on cultures is not limited simply to tales of myth and legend, though. Comets throughout history have been blamed for some of history's darkest times. In Switzerland, Halley's Comet was blamed for earthquakes, illnesses, red rain, and even the births of two-headed animals.
The Romans recorded that a fiery comet marked the assassination of Julius Caesar, and another was blamed for the extreme bloodshed during the battle between Pompey and Caesar. In England, Halley's Comet was blamed for bringing the Black Death. The Incas, in South America, even record a comet having foreshadowed Francisco Pizarro's arrival just days before he brutally conquered them.
Comets and disaster became so intertwined that Pope Calixtus III even excommunicated Halley's Comet as an instrument of the devil, and a meteorite, from a comet, became enshrined as one of the most venerated objects in all of Islam.
Although comets are not uncommon in our solar system, Great Comets only appear once a decade on average . There's is no major significance about the size of "Great Comets", rather they are simply defined as comets that become exceptionally bright and can be seen by casual observers with the naked eye. This usually has to do with their distance from Earth ( via activistpost.com ).