Brocéliande is the name of a mystical forest that first appeared in the writings of Wace.
Chrétien de Troyes mentions the forest in his romance, The Knight with the Lion. While in Brocéliande, Yvain pours water from a spring into a stone, causing a violent storm to erupt. This in turn summons the knight Esclados le Ros who defends the forest.
Robert de Boron associates the wizard Merlin with Brocéliande in his poem Merlin, also known as the Estoire de Merlin, or the Vulgate or Prose Merlin. It is where he is magically imprisoned by his love, either Nimue, Vivian, or sometimes Morgan le Fay. The prison usually takes the form of an oak tree or of a cave.