Geoffrey of Monmouth, a cleric probably of Welsh or Breton origin, completed his Historia Regum Britanniae around 1138, presenting a sweeping legendary history of Britain from its Trojan founders to the Saxon invasions. The work gave literary form to the legend of King Arthur, tracing his birth at Tintagel, his magical sword, his court at Caerleon, and his final departure to Avalon. Though written in Latin, it was immediately translated into French by Wace and into Middle English by Layamon, igniting a literary tradition of Arthurian romance that would define European vernacular literature for centuries.

Geoffrey's Historia was the fountainhead of Arthurian literature, directly inspiring Chrétien de Troyes, Wace's Roman de Brut, Layamon's Brut, and ultimately Malory's Morte d'Arthur and Tennyson's Idylls of the King. The Merlin figure Geoffrey elaborated became a recurring archetype in poetry and drama. The text also helped legitimise vernacular storytelling as a vehicle for serious historical and moral content, opening the door for later poets writing in French and English.