In the first section of the first branch of the Mabinogi, Pwyll is guiding a pack of dogs through a forest, and encounters a pack of white dogs with red ears killing a stag. Magical white animals from Annwfn were a common motif across these medieval celtic stories, so these white dogs would likely be obvious to the intended audience as magical. The appearance of these strange dogs tell the reader that Pwyll has accidentally entered an otherworld. This otherworld is treated much differently than others, though. It is not overwhelmingly good or evil, or home to divinity, like most seem to be, but rather just a world with slightly different rules to our own. The conflict in the first section is set up to be that Pwyll must defeat the ruler of the only kingdom in Annwfn other than Arawn’s (to pay off a debt incurred by scaring off Arawn’s special dogs so his own could feast on the stag), a kingdom which his own is constantly at war with. This tells the reader that Annwfn is not perfectly utopian, or completely hellish.
It is not a land which resides outside of the established world, but a diplomatically valid entity (with comparable political structure to Dyfed), which one can easily access (even by accident) without need for ritual or upfront magic. In Annwfn, things work slightly differently, though. The reader is (after the dogs, which our culture may not instantly read as magical) confronted with the Arawn’s ability to shapeshift Pwyll into himself and vice versa, in order to pull off some unnecessarily elaborate ruse. Pwyll must defeat the rival ruler Hafgan in a mortal duel. Arawn predicts (or maybe causes) that Hafgan will be defeated in a single blow, but that Pwyll must decline to finish him off or Hafgan would somehow survive and be healed by the next day. Here we see exemplified the classic tradeoff of power in fairy tales and the like, wherein a powerful magical being must enlist earthly help to solve their problem (in this case, the solution to the problem is literally given to the protagonist, and the reader must simply accept that the otherworldly ruler cannot execute on this himself for some reason). Magical problems require strange magical (superstitious?) solutions. The single blow instruction works out for Pwyll, and Arawn is surprised to find that Pwyll never slept with his wife for the one year he was shapeshifted (this text makes a motif out of having everything happen in intervals of one year), and their bond of friendship deepens. From this great friendship established between himself and Arawn he earns the title of “Ruler of Annwfn”. Later, in the second section, his future wife Rhiannon is also depicted as otherworldly. She is said to ride a white horse and wear gold-brocaded silk (the land of Annwfn is also described as having lots of gold-brocaded silk), and possess some sort of magical aura making her impossible to catch up with even on the fastest horse, only reachable once she decides to wait.
Normally, one would expect to find Annwfn a far off and unreachable land and Rhiannon inviting and stumbled upon by the protagonist. Instead, in a way that builds a nice bit of interest in the narrative, one finds the opposite is true. Annwfn is a strange example of an otherworld because instead of overturning and contrasting with reality, it operates as a non-divine mirror of the world of the author, albeit one that is hyperbolically prosperous with some magical elements.
Parker, W. M. “The Mabinogi of Pwyll.” Mabinogi.net, 2003, www.mabinogi.net/pwyll.htm.
Guest, Charlotte. The Mabinogion. 2002, gutenberg.org/files/5160/5160-h/5160-h.htm.