Cadwaladr figures prominently in Geoffrey of Monmouth's romantic account of the "Historia Regum Britanniae" (). As such, the Cadwaladr of Geoffrey is a literary invention that used the name of a historical person to advance the plot of the story. In Book XII, Chapter XIV of the "Historia", Cadwaladr is given as the last in a line of kings that began with Brutus of Troy. Chapters XV – XVIII have him leaving a depopulated Britain for Brittany, where the British people have resettled. Britain itself has been almost emptied by plague; for eleven years the country was "completely abandoned by all the Britons" except for parts of Wales. Cadwaladr is received as a guest by Alan Hir, King of Brittany. Taking advantage of the depopulation, the Saxons invite more of their countrymen to join them as soon as the plague abates. From this point they become completely dominant in Britain, and the British come to be called the "Welsh".