Arawn

"You have long lost the benefits of your Destiny, now you are at the mercy of the Gods that crafted it."

- Arawn to Merlin and Arthur.

Arawn is the God of the Overworld Realm known as "Annwn" - a realm that is home to both the dead and the Kingdoms of Fae and Sidhe, the Lake of Avalon being one of many locations that cross between the living and overworld Realms, although Arawn is a spiritual entity like the other deities that cannot cross into the Living World of his own volition.

Arawn requires a Vessel of from the Pen Annwn family bloodline and every few centuries a youth is born from the family and grows in his image, carrying a piece of Arawn's soul and unaware of their destiny until the given time, when Arawn requires to appear in the Human realm for whatever threat.

Earliest Sightings
The origins of Arawn are unknown except that he was the God of the Dead and ruled the Overworld, one of the leading deities of Celtic myth, Arawn was legendary as one of the

Trivia

 * In Welsh mythology, Arawn was the king of the otherworld realm of Annwn, appearing prominently in the first branch, and alluded to in the fourth.
 * Arawn does not mention having a Wife in Merlin, but he does admit that his sister lives with him, showing differences to the original mythology.