Although founding dreams are a worldwide tradition in the chronicles of the Middle Ages, they have not taken attention enough. This article shows that, as a founding dream, the dream of William the Conqueror's mother is firstly crated by William of Malmesbury influenced by Classics and the dream interpretation tradition coming through Greeks. Later, Wace and Benoit, by preserving its frame, rewrite the dream in a way of which is more understandable to the twelfth century European common man. This article will uncover evidences through dream interpretation sources, mainly Artemidorus, and medieval European cultural figures, mainly Tree of Jesse. This is a possible scenario for how the dream of Herleva was created and developed.