Breuddwyd Rhonabwy or 'The Dream of Rhonabwy', is one of two independent Welsh Arthurian tales included in a late fourteenth century work called the Red Book of Hergest, (The other being Culhwch ac Olwen.) that is now considered part of the Welsh story cycle known as the Mabinogion.
The story tells of a dispute between Madog ap Maredudd, king of Powys and his brother Iorwerth, and in particular of Rhonabwy, a warror in the service of Madog, who takes part in the search for the rebellious Iorwerth, and falls asleep to dream of a time when Arthur is busy assembling a host to fight a great battle. The bulk of the narrative deals with the events of the dream and contains the usual mnemonic repetition of incidents and charcters.
Now Madog ap Maredudd was a genuine twelfth century king of Powys, and it is therefore believed that the tale was first written down sometime during Madog's reign. Since the tale also features other, much earlier historical characters such as Rhun ap Maelgwyn, Selyf ap Cynan and Owain ab Urien it is very likely that its core is a genuine record of a much earlier Welsh Arthurian tradition, with the dream itself simply placed in a contemporary setting and given a medieval veneer.
What is interesting about the story is that it shows us an Arthur moving towards 'Mount Badon' for the battle of Mons Badonicus against a gentleman known as Osla Gyllellfawr, 'Osla of the Long Knife'. And yet it is clear from the narrative that the Battle of Camlann is at least seven years in the past, and Arthur seems quite healthy and very much alive. This is a curious inversion of the normal Arthurian tradition that places 'Mount Badon' or Mons Badonicus, well before Arthur's death at Camlann. It is almost as if Rhonabwy's dream constitutes a visit to the otherworld, where Arthur and his men exist in some kind of pereptual limbo, where time is circular and they are continually fighting and re-fighting one battle after another.
Which just goes to show how little reliance can be placed on tradition in the construction of history if nothing else.
The text that follows is essentially the 1849 translation by Lady Charlotte Guest, which I have taken the liberty of 'modernising' and adding explanatory notes where necessary.
The Dream of Rhonabwy I
The Dream of Rhonabwy II
The Dream of Rhonabwy III