The earliest surviving account of the Saxon invitation comes from the sixth-century writer Gildas, who does not name Vortigern. He condemns a 'proud tyrant' and a council that invited Saxon mercenaries to defend Britain after Roman rule had collapsed. The soldiers arrived in a small force, received supplies and land, then demanded more. When relations broke down, the allies became enemies. Later writers identified the unnamed tyrant with Vortigern and built a personal tragedy around the political disaster.

The Historia Brittonum, compiled in the ninth century, gives the ruler the name Guorthigirn and expands his dealings with the Saxon leaders Hengist and Horsa. Hengist, a warrior remembered as one of the founders of early English settlement, gains influence at the British court and brings more followers across the sea. The story gives Vortigern a marriage to Hengist's daughter, later called Rowena, and links that marriage with the surrender of territory in Kent. The same tradition remembers Vortimer, Vortigern's son, as a British war leader who attacks the Saxons and temporarily drives them back.

The same narrative tradition gives Vortigern one of Britain's most famous prophetic episodes. Seeking a secure stronghold in the mountains of Wales, he tries to build a fortress at Dinas Emrys. Each day's work collapses during the night. Advisers demand the blood of a fatherless boy, but the chosen child exposes their ignorance and orders the foundations opened. Below the hill lie a pool and two dragons, one red and one white. When released, the dragons fight. The red dragon is interpreted as the Britons and the white dragon as the Saxons, turning a failed building project into a prophecy about the struggle for Britain.

Geoffrey of Monmouth, writing in the twelfth century, reshapes the fatherless prophet into Merlin and gives Vortigern an even fuller political career. In this version Vortigern raises the monk Constans to the throne after the death of King Constantine, rules through him, and eventually takes power himself after Constans is murdered. Constantine's younger sons, Aurelius Ambrosius and Uther Pendragon, escape overseas and later return as enemies. Geoffrey's Vortigern is therefore not simply the man who invites the Saxons, but a usurping ruler whose earlier choices return against him.

Another major episode is the Treachery of the Long Knives. Hengist invites the British leaders to a peace meeting while his followers conceal knives beneath their clothing. At a signal the Saxons kill the unarmed Britons around them. Vortigern is spared but loses control of the realm he tried to preserve. The massacre belongs to later legend rather than secure fifth-century history, yet it became one of the defining explanations for how a hired war-band could become a conquering people.

Vortigern's death changes between texts. The Historia Brittonum associates him with a fiery end, while Geoffrey has Aurelius Ambrosius pursue him into Wales and burn the fortress in which he has taken refuge. Across those versions the conclusion is consistent in spirit: the ruler who tried to protect himself through shifting alliances dies trapped by the consequences of those alliances. His name survives beside Hengist, the red and white dragons and Dinas Emrys as one of the great cautionary figures of post-Roman British legend.