He's the son of the goddess Dôn, and brother of the sorcerer Gwydion, the goddess Arianrhod, and the smith-god Gofannon.
Notable Stories Involving Him:
He stole a dog, a bird, and a deer from Arawn, god of the Otherworld (the afterlife), leading to a war between Arawn and the Children of Dôn (the Welsh counterparts to an Irish tribe of gods known as the Tuatha De Danann). Thankfully, his brother Gwydion used his magic to turn trees into warriors (hence why the battle is referred to as the Battle of the Trees), helping the Children of Dôn defeat Arawn and his forces.
SOURCES: Wikipedia, Encyclopedia Mythica, & GodChecker