From the Third Branch of Y Mabinogi
[Manawydan and Pryderi are following a shining white boar which leads them to an empty fort into which the boar has disappeared. Pryderi wants to follow. Manawydan says]:
“It is not a good idea to go into the fort. We have not seen the fort here before and my advice is not to go near it.” But Pryderi replies, “I will not abandon my hounds” and, in spite of Manawydan’s counsel, goes to the fort.
[…..]
After waiting to see if Pryderi will return, Manawydan returns home without him.
When he comes in, Rhiannon looks up and asks “Your companion and the dogs, where are they?” He told her the tale. “Indeed, you were a poor companion, and it’s a good companion that you have lost”. With that word she went off to where he said the fort was.
She saw that the gate was open, and it was not hidden. In she came and discovered Pryderi grasping a basin [attached to a fountain] and said “Oh my lord, what are you doing here?” She took hold of the basin. As soon as she does this, her hands also stick to the basin and her feet to the base of the fountain, and she is struck dumb.
As soon as night fell, behold, a great tumult, a shower of mist, and the fort disappears with them in it
