Around 1134–1136, Geoffrey of Monmouth completed his Historia Regum Britanniae ('History of the Kings of Britain'), a pseudo-historical narrative presenting a legendary history of Britain from Brutus of Troy through the reign of Cadwaladr, with extended attention to the reigns of King Lear, Cymbeline, and above all King Arthur. Written in Latin and presented as a translation of a lost 'ancient book,' the Historia proved phenomenally popular across Europe. It fundamentally shaped how medieval Europeans understood British identity, Arthurian legend, and the place of Britain in world history.

Geoffrey's Historia is arguably the single most consequential source text for medieval and Renaissance English-language poetry. It provided the raw material for Wace's Roman de Brut, Layamon's Brut, Malory's Morte d'Arthur, and the entire Arthurian tradition that runs through Tennyson's Idylls of the King and beyond. His portrayal of Merlin as prophetic poet-sage also shaped the archetype of the visionary bard central to later British literary self-understanding, from Spenser to William Blake.