The First Branch of the Mabinogi gives Annwn its clearest prose setting. While hunting, Pwyll drives away a strange pack of white hounds with red ears and claims their kill. Their owner is Arawn, king of Annwn. To make amends, Pwyll agrees to exchange appearance and position with Arawn for a year and a day. He rules in Arawn's place, treats Arawn's wife honourably and then meets Hafgan, Arawn's rival, in single combat. By following Arawn's instructions and refusing Hafgan a second blow, Pwyll defeats him. When the two men resume their own forms, their friendship links Dyfed and Annwn, and Pwyll becomes known as Pwyll Pen Annwn.

Annwn is also central to Preiddeu Annwfn, The Spoils of Annwn, a difficult poem preserved in the Book of Taliesin. The poem describes an expedition led by Arthur in his ship Prydwen and repeatedly laments that, apart from seven, the warriors do not return. It moves through a series of otherworldly strongholds whose names include Caer Sidi and Caer Pedryfan. One of its most famous images is the cauldron of the Chief of Annwn, dark around its rim and set with pearls, kindled by the breath of nine maidens and refusing to boil the food of a coward. The poem is highly allusive, so modern summaries should avoid pretending that every fortress and event can be reconstructed as a straightforward adventure narrative.

Different Welsh traditions associate different supernatural rulers with Annwn. Arawn is explicitly king of Annwn in the First Branch. Gwyn ap Nudd is connected in later material with supernatural beings, the fair folk and otherworldly rulership; in Culhwch and Olwen he is charged with restraining destructive spirits. These traditions are better understood as overlapping bodies of myth than as evidence for a literal sequence in which Gwyn simply replaced Arawn on a throne. The Cŵn Annwfn, the otherworldly hounds whose white bodies and red ears echo Arawn's hunting dogs, became another enduring feature of Welsh supernatural folklore.

Christian literature sometimes recast the Otherworld in more hostile terms. In the story of Saint Collen, Gwyn ap Nudd appears in a magnificent supernatural court on Glastonbury Tor, and Collen dismisses the splendour as demonic glamour before sprinkling holy water and causing the vision to vanish. That episode reflects a Christian interpretation of older supernatural traditions rather than a single authoritative definition of Annwn as Hell. Earlier Welsh material can present the Otherworld as dangerous, wealthy and marvellous without reducing it to a place of punishment.

Annwn later became important to discussions of the development of Arthurian quest literature because the cauldron in Preiddeu Annwfn resembles other Celtic vessels of abundance and has sometimes been compared with the later Holy Grail. That connection remains an interpretation rather than a proven line of descent. The strongest evidence stays within the Welsh texts themselves: Arawn's kingdom, Pwyll's temporary rule, Arthur's perilous voyage in Prydwen, the cauldron of the Chief of Annwn and later associations with Gwyn ap Nudd. Together they make Annwn a distinctly Welsh mythological location rather than a historical place on an ordinary map.