How does the "mighty man of magic" atone for his plague? In _Lludd and Lleuelys_, three plagues fall upon Britain. The first consists of a fierce race, the Coranians; the second consists of a terrible noise which renders the Britons terrified, and the third is a plague of disappearing food and drink. Lludd, after council with his brother, Lleuelys, king of France, figures out how to end the first two plagues, and then fights the "mighty man of magic" who is responsible for the third, defeating him. The magician pleads for mercy. > All the losses that ever I have caused thee," said he, "I will make thee atonement for, equal to what I have taken. And I will never do the like from this time forth. But thy faithful vassal will I be." And the king accepted this from him. How does he atone for his deeds? The tale ends shortly after, and I cannot find another story in which it continues. I suspect that this detail is never revealed, but I could be wrong.

The answer is in the quote:

> All the losses that ever I have caused thee," said he, "I will make thee atonement for, **equal to what I have taken**. And I will never do the like from this time forth. **But thy faithful vassal will I be**." And the king accepted this from him.

Presumably, the atonement consists of this man giving Lludd the food back (or something equivalent in value), and of becoming Lludd's vassal.

The magician isn't mentioned anywhere else in the Welsh cannon, so this is as much information as you are going to get.