Procopius, a sixth century Byzantine writer who had some dealings with English emissaries, took an interest in the distant island of Britannia; and he was somewhat daunted. In the west of the island, beyond the wall of Severus, lies a land of serpents, vipers and wild beasts, where the souls of men are brought, he declares. He had certainly his Geography wrong, as he confused the area with Scotland and he wrongly located Hadrian's wall, but the place where the Celts thought that men's souls were brought was the Berwyn Moorlands, on the borders of Wales, where the ancient Britons believed that the gateway to Annwn, [pronounced Annuvin] the realm of the dead, lay. On these sparse moorlands, the Cwn Annwn, the hounds of Annuvin. hunted,seeking souls of the dead roaming in the mist to hound into the underworld.
Look at the image below and enjoy Pistyll Rhaeadr, one of the seven wonders of Wales. This is a beautiful waterfall that drains the moors on Moel Sych towering above it, but for the Celts it was the gateway to a mysterious land. Not for nothing were the moors known as Rhos y Beddau, the moors of graves. Annuvin lay somewhere up there, and the moorlands contained deadly traps, bogs into which you could sink chest deep. This was a land that featured in the myths of Arthur, a Welsh warlord who was at home in that region, to the extent that one of the places on the Berwyn Ridge is known as Arthur's Table [Bwrd Arthur.]
The Welsh were so convinced that this was a magical place that the legend that travellers across the moor could be met by Gwynn ap Nudd, a Celtic underworld deity believed especially at large at Samhain [modern Hallowe'en.] developed. He would offer them food, but if they took it they were swallowed up into his underworld, never to escape. There is a legend that Gwynn once on the Berwyn Moors tried to capture St Coll, but Celtic saints were used to this kind of thing, so he resolutely refused, and then saw Gwynn's palace fade away, leaving him alone on the moor. He became the patron saint of nearby Llangollen.
Don't be unnecessarily alarmed. There is good walking on the main paths, especially along the ridge. I have only once put my foot into a bog hole, and as normally is the case I was wearing light coloured trousers at the time. I never go into mud except when I am wearing light clothes. Some mischievous sprite is behind this, I suspect. ,