Math ap Mathonwy
: MAHTH ap mah-THON-wee
Welsh: Rich son of Treasure?
Welsh god of magic and the underworld (or more the riches of the underworld, as opposed to the souls there-in. In the family of the Children of Don, that position goes to Nudd/Lludd and his sons. He is the brother of the goddess Don and has no offspring.
He appears most prominently as a the Maimed King of Gwynedd (Northern Wales), whose feet must be supported by a maiden. When his maiden Goewin is raped by his nephew Gilfaethwy, he punishes the nephews (Gilfaethwy and Gwydion). Gwydion is punished because he engendered a war with Pryderi so that Gwydion could steal the prince's magic pigs and create a distraction so that Gilfaethwy could rape Goewin. Math turned his nephews into deer, boars, and wolves, and made them mate with each other. Then, needing a new maiden, Arianrhod is suggested, but she fails the test with his magic wand. He also creates a woman out of flowers for his grandnephew Lleu to marry.
This is all recounted in both the Mabinogion's tale "Math fab Mathonwy" and Taliesin's poem "Cad Goddeu--the Battle of the Trees."