Prydwen

Prydwen plays a part in the early Welsh poem "Preiddeu Annwfn" as King Arthur's ship, which bears him to the Celtic otherworld Annwn, while in "Culhwch and Olwen" he sails in it on expeditions to Ireland. The 12th-century chronicler Geoffrey of Monmouth named Arthur's shield after it. In the early modern period Welsh folklore preferred to give Arthur's ship the name Gwennan. "Prydwen" has however made a return during the last century in several Arthurian works of fiction.
"Preiddeu Annwfn".
Arthur's ship makes an early appearance in "Preiddeu Annwfn" ("The Spoils of Annwn"), a Welsh mythological poem of uncertain date (possibly as early as the 9th century or as late as the 12th) preserved in the Book of Taliesin. The meaning of the poem is in many places obscure, but it seems to describe a voyage in "Pridwen" to Annwn, the Celtic otherworld, to rescue a prisoner held there. It includes two lines translated by John K. Bollard as
And again later
A more literal translation of the first phrase is "three fullnesses of "Prydwen"", but it is not clear whether we are to understand this as representing three voyages by "Prydwen", a single voyage of a threefold-overloaded "Prydwen", or a flotilla of three ships each of which contains as many men as would fill "Prydwen".
"Culhwch and Olwen".
"Prydwen" appears in three episodes of the tale "Culhwch and Olwen", which reached its final form c. 1080–1100. First Arthur goes to sea in "Prydwen" in an attempt to capture the bitch Rhymhi and her cubs. Then he and a small force sail in "Prydwen" to Ireland, take the cauldron of Diwrnach as booty, and sail back to Wales. Finally Arthur and all the warriors of Britain return to Ireland in search of the boar Twrch Trwyth, and when the boar and his piglets swim to Wales they follow him in "Prydwen".
Geoffrey of Monmouth.
In Geoffrey of Monmouth's "Historia Regum Britanniae", written in the 1130s, he listed Arthur's weapons, giving his shield the name Pridwen. His reason for doing that is uncertain, but it may be that he thought a name meaning "fair face" was appropriate for a shield which, he says, was adorned with an image of the Virgin Mary. It has also been suggested that Prydwen, as a magical object in Welsh tradition, could be both a shield and a ship.
In popular tradition.
Further evidence for the early existence of the "Prydwen" tradition comes from a document in the 12th-century Liber Landavensis which records the place-name "messur pritguenn", "the Measure of Prydwen". 
In one 16th-century manuscript (BL, Add. MS. 14866) Caswennan, the name of a sandbank in Gwynedd, is glossed as "a place hateful to ships, near Bardsey and Llŷn; there Arthur's ship named "Gwennan" was wrecked". In 1742 the hydrographer and antiquary Lewis Morris found that the same tradition was still current in that locality. The poet Evan Evans repeated this story in 1764, but made "Caswennan" the name of the ship. Iolo Morganwg (1747–1826), antiquarian and forger, listed seven of the ships belonging to King Arthur which "conveyed the saints to Ynys Enlli". He included "Gwennan" but not "Prydwen"; the other six names were purely fanciful. In other sources the ship "Gwennan Gorn", wrecked on Caswennan, is said to have belonged not to King Arthur but to prince Madog ab Owain Gwynedd.
Modern appearances.
John Masefield's poem "The Sailing of Hell Race", in his "Midsummer Night and Other Tales in Verse" (1928), tells a story based on "Preiddeu Annwfn", though Arthur's ship is here called "Britain". Alan Lupack surmises that this is a play on the names "Prydwen" and "Prydain", the Welsh name for Britain.
In H. Warner Munn's 1939 novel "King of the World's Edge" Arthur and companions cross the Atlantic in "Prydwen". Susan Cooper's "Silver on the Tree" (1977), the last of her five Arthurian novels for children, ends with King Arthur sailing into the beyond in his ship "Pridwen". Guy Gavriel Kay's novel "The Wandering Fire" (1986), the second of his Fionavar Tapestry sequence, features "Prydwen" in another quest for a magical cauldron; the third and final novel, "The Darkest Road" (1986), ends as "Prydwen" carries Arthur, Guinevere and Lancelot away over unearthly seas. In Patricia Kennealy-Morrison's "The Hedge of Mist" (1996), the last novel in a science-fiction Arthurian trilogy, "Prydwen" is one of Arthur's spaceships.
Heather Dale's song "The Prydwen Sails Again", on her 1999 album "The Trial of Lancelot", again puts "Prydwen" into the context of the voyage to Caer Siddi.
The computer game "Fallout 4" presents "Prydwen" as a dieselpunk airship.