You are hereArawn
A Welsh god who is the ruler of the underworld. He is mentioned in the Mabinogion in the Tale of 'Pwyll Lord of Dyfed' and in 'The Spoils of Annwn'.
Pwyll meets the god while he out hunting on the fringes of his kingdom, and offends Arawn by letting his hounds loose on a stag already being hunted by him.
Eventually they enter into a bargain were they exchange places for a year in each others form. Pwyll becomes Lord of the Underworld, and Arawn becomes the Lord of Dyfed. During this time Pwyll slays Hafgan an enemy of Arawn who has been troubling his kingdom. They both become firm allies because neither of them sleeps with the others wife during their exchange.
Arawn was also the owner of a magical cauldron with rejuvenating powers, that would not boil a coward's food. Arthur led a quest into Annwn, Arawn's Kingdom, to steal this precious artefact. Arawn is also the guardian of the magical animals within his kingdom.
In many ways Arawn is a very similar character to Gwynn ap Nudd, they are both leaders of the wild hunt and lords of the underworld. Their pack of hounds share the same characteristics, and Gwynn ap Nudd also takes part in exchanges with other people. It is likely that they are two names for the same archetype.