Around 1138, Geoffrey of Monmouth wrote his Historia regum Britanniae, the first account of the Arthurian legend. It includes the dark, image-laden sayings of the wizard and prophet Merlin. Some of these sayings address the past of the British Isles, while others are prophecies about their future. Immediately, there was a need to interpret them. Among the numerous commentaries, the Explanatio in prophetia Merlini Ambrosii (circa 1170) is the most comprehensive. It also interprets the prophecies concerning the future of that time. The author of this extensive work is believed to be a certain Alanus, originally from Lille, who later became the Bishop of Auxerre. Specifically, it addresses the Anglo-Norman monarchy—also in relation to France—dynastic struggles, and the relationship between church and secular power. The customs of the time are criticized: court life, clerical marriage, as well as sorcery and superstition. The text, previously only available in 17th-century prints, is critically edited for the first time in this work, translated into modern language, and commented on with regard to its historical, intellectual, and literary significance.