Comet

Citing the "Book of Lore":

"...Over the ages, comets, fiery nebulae, have torn through our skies as harbringers of impending doom. They appeared just before the emergence of the Dark Lord Mondain.  They heralded the reign of the Enchantress Minax, and they foretold the impending danger of the hellborn Exodus."

In Ultima V, a comet seen in the Spyglass over one planet of the Britannian Solar System meant, that the city with the virtue the planet was named after, was under attack from one of the Shadowlords. With the destruction of the Shadowlords, the comets would vanish as well.

Now, with the threat of the Guardian looming over Britannia, the comets have begun again...