Annwn is the otherworld of Welsh mythology, most fully described in the First Branch of the Mabinogi, the medieval Welsh prose tale collection, where the mortal lord Pwyll trades places with Arawn, a king of Annwn, for a year, ruling Arawn's realm in his stead while Arawn rules Pwyll's own kingdom of Dyfed, a swap that ends with the two forming a lasting alliance rather than any conflict between the mortal and otherworldly kingdoms.
Annwn is described in the Welsh sources not as a place of punishment or judgment but as a genuinely desirable realm, associated with abundance, eternal youth and a cauldron of plenty, and later medieval poetry, including material connected to the bardic tradition surrounding Taliesin, describes a legendary raid on Annwn to seize this cauldron, a narrative some scholars connect to the wider Celtic and later Arthurian motif of a quest for a magical vessel of abundance or resurrection. Unlike the more clearly underworld-coded Welsh term uffern, used for a hell-like punishing realm in later, Christian-influenced Welsh literature, Annwn in its earlier medieval form reads closer to a parallel, desirable Otherworld than to a place of the dead specifically.