Notable Stories Involving Him:
He reigns over the underworld, called the Otherworld or Annwn, though his responsibility is sometimes attributed to the fairy king Gwyn ap Nudd in Arthurian mythology. Once, though, Arawn forced a prince named Pwyll to trade places with him for a year and a day, during which time the prince defeated a rival underworld deity, Hafgan. Since this was something Arawn had been having trouble doing, and because Pwyll had refused to sleep with Arawn's wife, he and the prince became good friends afterwards.
Arawn possesses hell-hounds that are said to be all white, except for having red ears. They ride through the skies in the colder months of the year, rounding up wandering spirits for their master. However, Arawn used to also have in his possession a dog, bird, and deer that were stolen from him by Amaethon, god of agriculture, directly leading to war against Amaethon and his brother Gwydion, which he lost.
SOURCES: Wikipedia, Encyclopedia Mythica, & GodChecker