" From the plotting of strangers and iniquitous
Monks, as the water flows from the fountain,
Sad and heavy will be the day of Cadwallon.

The lines come from the Red Book of Hergest, a collection of Welsh poems written in the late-fourteenth century but containing material that is much older.
This brings us, neatly, to J. R. R. Tolkien. For according to a learned authorial conceit, the source of his tales of Middle-earth was the Red Book of Westmarch. Tolkien was the Rawlinson and Bosworth Professor of Anglo-Saxon at Oxford University and one of his aims was to create a mythology for England, as the Red Book of Hergest, which contains the Mabinogion and other material, could be said to preserve the mythology of the Britons.

Many if not all the writers and scholars involved in Anglo-Saxon studies first came to the field through reading the professor’s stories – and I am one of them, so it is no accident that this story is called Oswald: Return of the King, in tribute and homage. Tolkien writes of Oswald in his seminal essay Beowulf: The Monsters and the Critics and the parallels between him and Aragorn – rightful king in exile returning to claim the throne – are obvious. "

― Edoardo Albert , Oswald: Return of the King (The Northumbrian Thrones, #2)