Einhard. The Life of the Emperor Charles. Chapter 20.
By one of his concubines he had a son, handsome in face, but hunchbacked, named Pepin, whom I omitted to mention in the list of his children. When Charles was at war with the Huns, and was wintering in Bavaria, this Pepin shammed sickness, and plotted against his father in company with some of the leading Franks, who seduced him with vain promises of the royal authority. When his deceit was discovered, and the conspirators were punshed, his head was shaved, and he was suffered, in accordance with his wishes, to devote himself to a religious life in the monastery of Prüm. A formidable conspiracy against Charles had previously been set on foot in Germany, but all the traitors were banished, some of them without mutilation, others after their eyes had been put out. Three of them only lost their lives; they drew their swords and resisted arrest, and, after killing several men, were cut down, because they could not be otherwise overpowered. It is supposed that the cruelty of Queen Fastrada was the primary cause of these plots, and they were both due to Charles' apparent acquiescence in his wife's cruel conduct, and deviation from the usual kindness and gentleness of his disposition. All the rest of his life he was regarded by everyone with the utmost love and affection, so much so that not the least accusation of unjust rigor was ever made against him.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.