[W emrys=L ambrosius, pertaining to the immortals; (g)wledig, prince].
Also known as Eidiol. Shadowy but historical 5th-century British commander known by Gildas (d. 547) and Historia Brittonum (9th cent.) as Ambrosius Aurelianus. To Gildas Emrys was ‘the last of the Romans’, in that he led Romanized Britons against the Saxon invaders under Hengest. Recent scholarship asserts that there are two men named Emrys, possibly father and son, whose stories were conflated. The elder Emrys struggled for power with Vortigern [W Gwrtheyrn] after 430, while Ambrosius the younger (Aurelianus) began a new British resistance that Gildas regarded as a continuation of the earlier. Emrys was thought to be at the Battle of Mount Badon, dated perhaps 475, 480, or 519.
Emrys (Ambrosius) is the name Historia Brittonum gives to the mysterious boy who confronts Vortigern in the building of Dinas Emrys [W Emrys/Ambrosius' fort]. Vortigern, retiring to the far reaches of his kingdom, tries to establish a new fortress in Gwynedd but is disconcerted by the collapse of the foundation each time workers try to construct it. He is advised to caulk the stones with the blood of a ‘fatherless boy’. When found, the clever boy astonishes the court magicians with his knowledge, and reveals that there is a pool under the foundations where two dragons battle. The movement of the two dragons, one red (i.e. Wales) and the other white (i.e. Saxon invaders), shakes the foundations. In retelling this story centuries later, Geoffrey of Monmouth in the Historia (1136) named the marvellous boy Ambrosius Merlin.
See Rachel Bromwich, Trioedd Ynys Prydain, rev. edn. (Cardiff, 1978), 345 ff.