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 Lady Charlotte Guest in the 19th century. It tells of the Welsh hero Lludd Llaw Eraint, 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 are the Coraniaid, who come to Britain and cannot be forced out because their hearing is so good that they can hear anything the wind touches. 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.
With the aid of a brass horn that prevents the Coraniaid from hearing their conversation, Llefelys offers solutions to each plague. The Coraniaid, he reveals, can be killed by a mixture made from a certain insect. This mixture is harmless to Britons, so Lludd must convene a meeting of both groups and throw the mixture over everyone, thereby destroying the invaders. The second plague is caused by a red dragon that is embroiled in combat with a foreign white dragon. Lludd must set a trap for them at the exact centre of the island called Oxford, put them to sleep with mead, and then bury them underground in a stone chest. The third plague is caused by a "mighty magician", who casts a spell to make the whole court fall asleep while he raids their stores. Lludd must confront him, keeping himself awake with a vat of cold water.