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?