When using the spyglass (or a telescope) in Ultima V, a comet seen in the night sky over any 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 all three Shadowlords, these comets vanished as well.
...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. Now, while we live in the midst of peace and enlightenment, the comets have begun again.