The Mabinogion: Pwyll Lord of Dyved
14th century, Middle Welsh, National Library of Wales
King Arawn's terms are that they should exchange places and live as each other for a year and a day.
Pwyll Lord of Dyved is hunting in a forest one day when his hounds interfere with the dogs of another huntsman. These newly-encountered dogs are strange, Otherworldly animals and their owner seems no less strange and Otherworldly. He declares himself, rather truculently, to be King Arawn of Annwvyn, of Not-World; King Arawn of the Otherworld.
Pwyll, eager to make amends for the offense he has obviously caused to this other man by driving his dogs away from the stag they had brought down, agrees to the terms of friendship that King Arawn offers. These are, that they should exchange places and live as each other for a year and a day.
'You shall go into Not-World,' explains King Arawn, 'and there you shall sleep with the most beautiful woman you have ever seen. I will give you my exact likeness so that no one will know that you are not me, and I will go into your country looking like you and nobody will know that I am not you.'
The downside of this agreement for Pwyll is that, in exactly a year’s time, he, as King Arawn, must meet one of King Arawn’s enemies in single combat at a ford. He is to offer his adversary only a single stroke of his weapon. This should be enough – says King Arawn – and be sure that you don’t offer him any more blows than this, because I have struck him many times in the past and always he has returned the next day as fit and as healthy as he ever was!