Notes for Storm Descending: (by alphabetical order, not appearance in the story)
Caer Loyw - "The City of Light" (aka Glouchester); where Mabon ap Modron was imprisoned by persons unknown.
Cilgwri - the Welsh name for Wirral, a wilderness appearing in legend as a semi-mythical place. Sir Gawain rode through Wirral, a traitor knight fled there in the tale of Libeaus Desconus, and King Arthur’s knights visited Wirral while aiding Culhwch in his quest.
Claymore - This probably doesn't need a note, but here, Ianto uses a Scottish claymore (not a basket-hilted broadsword, to make that distinction), which is a heavy, double-edged, two-handed blade that is entirely wrong for the purported time period, but just go with it. (On an interesting note, the largest claymore on record is a sword measuring 2.24 m and weighing 10 kg. It was actually used, too.)
Culhwch and Olwen - To sum up, Culhwch resists his stepmother's attempts to pair him off with her daughter, and she curses him to be able to wed no one but Olwen, the daughter of the evil giant Ysbaddaden. Culhwch asks for his cousin Arthur's help, and they visit Ysbaddaden to ask for Olwen's hand. Ysbaddaden, however, doesn't want Olwen to marry, because he's cursed to die when she weds. Therefore, he assigns every suitor 39 impossible tasks that require a course in Old Welsh just to pronounce. With Arthur's help, Culhwch completes all the tasks, only to learn of the final quest - he must retrieve a razor, scissors, and comb (or a mirror, razor, and comb in some versions) from between the ears of Twrch Trwyth. Only the greatest hunter alive can find Twrch Trwyth, and that hinter is Mabon ap Modron, who was stolen from between his mother and the wall when he was only three nights old. (This story does have a happy ending; Ysbaddaden is killed and Culhwch and Olwen live happily ever after.)
Cŵn Annwn - Spectral hounds from Annwn, the Welsh Otherworld. Associated with death and the supernatural.
Falcons - I use falcons of all types in this story, and while they don't specifically appear with Mabon ap Modron, they've long been associated with hunters in general.
King Arthur's sleep on Avalon vs his reincarnation - Those familiar with the Merlin finale will know that, in a deviation from the legend, Arthur wasn't carried to Avalon with mortal wounds, to sleep until Britain needed its king. Rather, Merlin spends a few thousand years waiting for him to be reborn. I'm...taking a liberal interpretation of "needs its king" to account for things like Twrch Trwyth triggering his reincarnation.
Mabon ap Modron - Literally "Divine Son son of Divine Mother" and the Welsh personification of youth. He was kidnapped from his mother's side when he was three nights old, and imprisoned until rescued by several Knights of the Round Table during Culhwch's quest to win Olwen. As for the legends, they're rather divided. One has a version of him as a servant of Uther, another has him as a student of Merlin, another has him as prisoner of the Court of Joy (which I've tried to reflect by having Arthur rescue him while still a prince, having Mabon as a minor sorcerer, and the inclusion of the Wild Hunt as his kidnappers. It gets rather complicated, admittedly).
Matrona/Modron - Modron was the mother of Mabon, and an aspect of the goddess Matrona (or the goddess herself, depending).
Myddfai and Llyn y Fan Fach - A story about the Lady of the Lake (or a version of her) who fell in love with a farm boy. She agreed to stay with him, but on the condition that he never strike her more than three times. Like any fairy tale, she made him rich and prosperous, bore him many happy children, etc. However, he became complacent, and ignoring the conditions. The Lady packed up, took all of his prosperity, kissed her children farewell, and walked back into the lake, never to be seen again.
Ousel - a type of blackbird with a white bib on its chest.
Prynhawn da - "good afternoon"
Talking Animals - On the quest to find Mabon ap Modron, the knights consult the oldest living animals. In order, these are the Ousel of Cilgwri, the Stag of Rhedynfre, the Owl of Cwm Cawlwyd, the Eagle of Gwernabwy, and the Salmon of Llyn Llyw.
Twrch Trwyth - A murderer who was transformed into a vast, evil boar that could only be found and/or (according to the legend) killed by Mabon ap Modron. He was as smart as a man, as big as an elephant, with tusks as long as a small tree and as sharp as a knife. In the version I referenced most often, Mabon killed him with his spear, though others have him being driven/hurled after death into the sea by Cornwall.
White Stag The Arthurian hunt for the White Stag was never-ending, and is often used as a metaphor for mankind's spiritual quest. (Mabon caught it because he wasn't mortal; anyone else would be out of luck.)
Wild Hunt - There are versions of the Wild Hunt all over Europe, especially in Germanic areas and in Britain. I've mixed myths a bit here, making the hunters dead (Germany), but with fey influences (England), and having the Cŵn Annwn hunt with them (Wales).
Ynys Avallach - Literally "The Isle of Apples"; Avalon.