Lludd and Llefelys (Welsh: Cyfranc Lludd a Llefelys) is a Middle Welsh prose tale written down in the 12th or 13th century; it was included in the Mabinogion by Charlotte Guest in the 19th century. It tells of the Welsh hero Lludd, best known as King Lud son of Heli in Geoffrey of Monmouth's Historia Regum Britanniae, and his brother Llefelys.
The story begins as Lludd inherits the kingship of Britain from his father, Beli. Soon after, he helps his brother Llefelys marry the princess of France and become king of that country. Though Lludd's reign starts off auspiciously – he founds "Caer Lludd", later to become London, as in Geoffrey – before long three plagues disrupt the peace. The first plague is a race known as the Coraniaid, who come to Britain and cannot be forced out, as their hearing is so good that they can hear anything the wind catches. The second plague is a horrid scream that comes every May Day and causes all pregnant women in Britain to miscarry. The third plague involves disappearing provisions: no matter how much Lludd may put in his stores, it will have vanished over the course of the night. Lludd takes his fleet to France to ask his brother's advice.