I have placed the Knights' sleeping place on Anglesey, in one of the many Welsh places named Ogof Arthur/ “Arthur's Cave”. In the spirit of the Nasuverse, lots of elements are stolen from many other cave legends concerning Arthur, though the inclusion of the Round Table is was my own idea (I don't dare call it an invention, because I'm certain it's not).
Ynys Enlli ('Island in the Tides') is indeed fortunate enough to have a suitably mythical name for me to translate. Ditto its unique brand of apple tree, although cuttings have begun to be distributed to try and build up the strain. A cave on the hillside above the mother tree is legendarily Merlin's prison (as are hundreds of others around the British Isles, of course, just as many caves throughout the world are where dead kings sleep. Sometimes, the same cave will host both legends, because why not?). This neatly ties in to Avalon's association with apples; which is most easily summarised by telling you to look at
part of Avalon's Wiki page.
The entire staff-making sequence came from me noticing that Merlin had an unexplained staff in later scenes. I think its insertion is just awkward enough that you can tell, but I liked the idea of him taking a kind of revenge on his coffin enough that I kept it.
Derfel is a historical figure – a saint who probably ministered around the Llandderfel area. He is also entangled in Welsh Arthurian myth, although we have lost most of his parts of the tale. He is named as Derfel Cadarn – the Mighty – and in some traditions one of the seven knights to have survived Camlann, by virtue of his strength. Most famously in modern times, Bernard Cornwell (author of Sharpe and others) uses him as the protagonist in his interpretation of the Arthur myth: the connection to Merlin is drawn from there, though the connection to Bardsey/Ynys Enlli (and therefore one of the many places Merlin is supposed to rest) is not – he is one of the 20,000 saints supposedly buried there; he may have been an abbot at a monastery on the island; additionally the monastery's founder and first abbot, St. Cadfan, is supposed to have been his cousin.
Merlin's place as the Magician of the Second is my own conjecture, derived from all the future-based magecraft he seems to do. It's more than a little iffy, since Merlin seems to still be at least partly alive in Nasu canon, and Zelretch is nevertheless running around (and it seems to be the case that there's only one Magic-user at a time). I'm claiming both that Merlin doesn't count as a Magician whilst he's confined and unable to use the Second, and that I have authorial fiat.
(Whilst we're at it, Teleportation is part of
some Magic, but it's also not certain to be the Second. I kinda think it makes sense, given how much the Second fits into movement and travel, but it's arguable. For the sake of cleaving to canon, I used a work-around rather than direct teleportation. Seems plausible that you'd get to choose where you popped out in a parallel world, since it'd be a right pain in the backside otherwise.)
I got fed up enough with everyone's variant spellings on Llenlleawg that I just went to some online pictures of the Culhwch and Owen manuscripts (both the Red Book of Hengest and the White Book of Rhydderch – thanks to the National Library of Wales and our own Jesus College for having those images open) and checked through to have a look at the direct source. Such fun, given that w was inconsistently written as something resembling a 6, final g as c and the fact I
don't speak Welsh, let alone badly transmitted, mediaeval Welsh in cramped, stylised handwriting. Well, I tracked it down anyway.
In Culhwch and Owen, Llenlleawg grabs Arthur's sword, swings it once and kills all their opponents during the retrieval of Diwrnach's Cauldron. He seems to be an Irishman like Diwrnach, which is pretty interesting. Overall, a neat little character, more than a little obscure and bound up in the multi-kill aspect with the Nasuverse has kept with Excalibur. So he got included and (given the strictly individual functioning of Noble Phantasms) turned out to be a middle finger to Nasu's decision that Shirō can't project that sword.
Artoria has, of course, ended up in Trafalgar Square by Nelson's Column. Because why not?