Thanks! I love a lot of pagan symbolism and those primordial myths. Annwn is just skimming the surface for a few visual motifs really...
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The best reference for Celtic myth is probably the Mabinogion, although the stories it collects were most likely written down by Christian monks. I mostly google and explore as many different sources as possible because they all have slightly different takes on the material, so I couldn't recommend a single authoritative source. It's always nice to find an obscure retelling that includes some juicy new detail...