"Spoils of Annwfn" which Arthur and others made a long journey overseas to obtain. Gweir was imprisoned in Caer Sidi through the spite (or messenger?) of Pwyll and Pryderi, associated as lords and defenders of Annwfn.4 Arawn, Lord of Annwfn, was defeated by Amsethon, son of Don, at the mythic battle of Cath Godeu.5
The Mabinogi of Math, son of Mathonwy,6 tells of Gilvsethwy's love for Goewin, Math's "foot-holder." To help him his brother Gwydion resolved to cause war and told Math that swine, unknown before, had been sent to Pryderi in Dyfed by Arawn. He and Gilvsethwy, disguised as bards, set off to the court of Pryderi, who praised Gwydion for his songs, whereupon the latter asked for the swine, but was told that they must breed double their number ere they left the country. Gwydion now obtained them in exchange for twelve stallions and twelve greyhounds magically formed by him from fungus; but these soon turned again to their original shape, and Pryderi invaded Math's territory, only to be defeated and slain in single combat by Gwydion's enchantments. Gilvsethwy outraged Goewin during the battle, and when Math discovered this, he transformed the brothers first into a couple of deer, then into swine, and finally into wolves. In these forms they had animal progeny, afterward changed to human shape by Math. Math now found a new "foot-holder" in Arianrhod, Gwydion's sister, but she proved no virgin, and when Math caused her to pass under his magic rod, she bore twins, one of whom was taken by Math and called Dylan. When Gwydion brought the other, who had grown rapidly, to Arianrhod's castle, she refused to give him a name. Disguised as a shoemaker, Gwydion then arrived with the boy and made shoes for Arianrhod which did not fit. She went on board Gwydion's ship, produced by magic, and saw the boy shoot a bird. Not recognizing him, she cried, "With a sure hand (llaw gyffes) lieu shoots the bird," whereupon Gwydion revealed himself and said that she had named the boy, Lieu Llaw
