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. In later tradition, the role of king of Annwn was largely attributed to the Welsh psychopomp, Gwyn ap Nudd. However, Arawn's memory is retained in a traditional saying found in an old Cardigan folktale:

English Translation: "Long is the day and long is the night, and long is the waiting of Arawn"

In the First Branch of the Mabinogi, Pwyll mistakenly sets his hounds upon a stag, only to discover that Arawn has been hunting the same animal. To pay for the misdeed, Arawn asks Pwyll to trade places with him for a year and a day and defeat Hafgan, Arawn's rival, at the end of this time, something Arawn has attempted but has been unable to do. Arawn, meanwhile, takes Pwyll's place as lord of Dyfed. Arawn and Pwyll become good friends because when Pwyll wore Arawn's shape, he slept chastely with Arawn's wife.

A friendship between the two realms is retained long after Pwyll's death; in the fourth branch, Pryderi, Pwyll's son and lord of Dyfed has received a gift of otherworldly pigs from Arawn. These pigs are eventually stolen by Venedotian magician and trickster, Gwydion fab Don, leading to Pryderi's invasion of Gwynedd. In the ensuing war, Gwydion kills Pryderi in single combat.