How did Manawydan know that the bishop (Llwyd Cil Coed) was responsible for the imprisonment of Pryderi and Rhiannon?

Question

In the third branch of the Mabiogion, Manawydan and his companions, Pryderi, Rhiannon and Cifga come across a deserted caer. First Pryderi and then Rhiannon go to investigate. They are forced immobile and speechless by putting their hands on a golden bowl; the caer, along with them, then disappears.

Manawydan is plagued with other ills, including a pestilence whereby his crops are besieged by a horde of mice. He catches one and decides to hang it. He is met first by a scholar, a priest, and a bishop, each of whom offer increasing prices to free the mouse. Manawydan declines each one. The scholar and priest leave after his refusal, but the bishop raises the price he will pay.

The bishop eventually allows Manawydan to name his price, whereupon Manawydan replies,

'I will name it,' he replied 'the freeing Rhiannon and Pryderi.'

The bishop agrees, and eventually the two lost companions reappear.

How did Manawydan know that the bishop (actually named Llwyd Cil Coed, with ulterior motives) was responsible for the disappearance of his friends and the other plagues upon the foursome?

Answer 1

Throughout the story, it's very clear that one "power" is responsible for the events occurring to Manawydon and his friends:

'God knows,' said the other 'your counsel to go into the caer is not good. We did not see this caer here ever before. If you would follow my counsel, you would not go in. Whoever put enchantment on this land, has also made the caer appear here.'

(All quotes are from the source cited in the question)

Part of the enchantment is that the character's land becomes deserted, and when the bishop (and the priest and the scholar) visit, "It had been all of seven years since he had seen man or beast - other than the four people who had been together (until two had been lost)." Since the visitors are concerned with saving one of the creatures responsible for the devastation of Manawydon's land, it doesn't take a genius to guess that they had something to do with the other enchantments.

Don't forget that there's intuition going on here: these characters are in a magical setting and are familiar with magic.