Einhard. The Life of the Emperor Charles. Chapter 32.
Very many omens had portended his approaching end, a fact that he had recognized as well as others. Eclipses both of the sun and moon were very frequent during the last three years of his life, and a black spot was visible on the sun for the space of seven days. The gallery between the basilica and the palace, which he had built at great pains and labor, fell in sudden ruin to the ground on the day of the Ascension of our Lord. The wooden bridge over the Rhine at Mayence, which he had caused to be constructed with admirable skill, at the cost of ten years' hard work, so that it seemed as if it might last forever, was so completely consumed in three hours by an accidental fire that not a single splinter of it was left, except what was under water. Moreover, one day in his last campaign into Saxony against Godfred, King of the Danes, Charles himself saw a ball of fire fall suddenly from the heavens with a great light, just as he was leaving camp before sunrise to set out on the march. It rushed across the clear sky from right to left, and everybody was wondering what was the meaning of the sign, when the horse which he was riding gave a sudden plunge, head foremost, and fell, and threw him to the ground so heavily that his cloak buckle was broken and his sword belt shattered; and after his servants had hastened to him and relieved him of his arms, he could not rise without their assistance. He happened to have a javelin in his hand when he was thrown, and this was struck from his grasp with such force that it was found lying at a distance of twenty feet or more from the spot. Again, the palace at Aix-la-Chapelle frequently trembled, the roofs of whatever buildings he tarried in kept up a continual crackling noise, the basilica in which he was afterwards buried was struck by lightning, and the gilded ball that adorned the pinnacle of the roof was shattered by the thunderbolt and hurled upon the bishop's house adjoining. In this same basilica, on the margin of the cornice that ran around the interior, between the upper and lower tiers of arches, a legend was inscribed in red letters, stating who was the builder of the temple, the last words of which were Karolus Princeps. The year that he died it was remarked by some, a few months before his decease, that the letters of the word Princeps were so effaced as to be no longer decipherable. But Charles despised, or affected to despise, all these omens, as having no reference whatever to him.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.