The most significant appearance of Ambrosius is the story about Ambrosius, Vortigern, and the two dragons beneath Dinas Emrys, "Fortress of Ambrosius" in Chapters 40–42. In this account, Ambrosius is still an adolescent but has supernatural powers. He intimidates Vortigern and the royal magicians. When it is revealed that Ambrosius is the son of a Roman consul, Vortigern is convinced to cede to the younger man the castle of Dinas Emrys and all the kingdoms in the western part of Britain. Vortigern then retreats to the north, in an area called Gwynessi. This story was later retold with more detail by Geoffrey of Monmouth in his fictionalised Historia Regum Britanniae, conflating the personage of Ambrosius with the Welsh tradition of Myrddin the visionary, known for oracular utterances that foretold the coming victories of the native Celtic inhabitants of Britain over the Saxons and the Normans. Geoffrey also introduces him into the Historia under the name Aurelius Ambrosius as one of three sons of Constantine III, along with Constans and Uther Pendragon.
The most significant appearance of Ambrosius is the story about Ambrosius, Vortigern, and the two dragons beneath Dinas Emrys, "Fortress of Ambrosius" in Chapters 40–42. In this account, Ambrosius is still an adolescent but has supernatural powers. He intimidates Vortigern and the royal magicians. When it is revealed that Ambrosius is the son of a Roman consul, Vortigern is convinced to cede to the younger man the castle of Dinas Emrys and all the kingdoms in the western part of Britain Vortigern then retreats to the north, in an area called Gwynessi. This story was later retold with more detail by Geoffrey of Monmouth in his fictionalised Historia Regum Britanniae, conflating the personage of Ambrosius with the Welsh tradition of Myrddin the visionary, known for oracular utterances that foretold the coming victories of the native Celtic inhabitants of Britain over the Saxons and the Normans. Geoffrey also introduces him into the Historia under the name Aurelius Ambrosius as one of three sons of Constantine III along with Constans and Uther Pendragon