The Welsh collection of stories called The Mabinogion is one of the major surviving bodies of Welsh myths. The stories in their modern forms are derived from two Older Than Print medieval Welsh manuscripts, the White Book of Rhydderch and the Red Book of Hergest, along with a collection of smaller texts, but those are simply the oldest written versions of stories that are based on older oral legends; some motifs and plots can be traced back to the early Iron Age (1st millenium BCE). They are the product of a highly developed narrative tradition, both written and oral.
The name is the plural of Mabinogi somewhat archaic Welsh, but interestingly enough, the work should be called the "Mabinogi," since it consists of four branches of a single Mabinogi, rather than multiple ones. A scribe made a mistake in the first branch, Pwyll Pendefig Dyfed, when he referred to it as the Mabinogion, which he later rectified in the other branches, but the name stuck. The best translation of "Mabinogi" appears to be "tales of childhood". Mab is the Welsh word for "son" (it's from the same root as the Gaelic mac), and the consonant-mutation "Vabinogi" occurs in "Llyma Vabinogi Iesu Grist", a medieval manuscript (Peniarth MS 14) describing the childhood of Jesus.
The work is divided into four Branches, five Native tales, and three Romances.
The Four Branches create an Arc that follows the mythological heroes of Welsh pre-Christian mythology. The branches are connected by the struggles of the children of Llŷr and the life of Pryderi. They are, in order:
- Pwyll, Prince of Dyfed, detailing the journey of Pwyll into the kingdom of Annwn, the Celtic Otherworld. After returning to his own kingdom, he meets and wins his wife, the enchanted Rhiannon, from her unwanted suitor Gwawl. Pryderi is born, an event that initially brings more suffering than joy to his parents when he disappears mysteriously.
- Branwen, Daughter of Llŷr, is about the marriage of Branwen, sister of the king Bendigeidfran (Brân the Blessed), to the king of Ireland in a failed attempt to bring peace. The wrathful temper of her brother Efnysien starts a chain of events that ultimately destroys both kingdoms. Only seven Welsh soldiers survive the war, including Pryderi. The events of this branch lead directly into the next.
- Manawyddan, Son of Llŷr, in which Branwen's brother Manawyddan marries the now-widowed Rhiannon. Together with Pryderi and his wife Cigfa, they struggle against a series of disasters and curses perpetrated by Llwyd ap Cil Coed, friend of Gwawl from the first branch.
- Math, Son of Mathonwy, is nominally about the magician-king of Gwynedd, but most of the action centers around his nephew Gwydion fab Dôn for some reason.
- Gwydion first engineers a war - the war in which Pryderi is slain by Gwydion - to lure Math away from his castle so that Gilfaethwy, Gwydion's brother, can sneak into Math's stronghold and rape his foot-holder maiden, Goewin. In punishment, Math transforms the brothers into a different animal every year, one male and one female, until they bear three offspring together.
- Still up to mischief, Gwydion then suggests Goewin be replaced by his sister Arianrhod. Math puts her through a magical virginity test of stepping over a rod, causing her to promptly give birth. The child is largely shoved off screen but Arianrhod also leaves a scrap of flesh that Gwydion keeps in a box and somehow becomes another boy.
- The rest of the narrative deals with Gwydion raising said boy and basically outwitting (or out-magicing) his sister's curses to deny Lleu Llaw Gyffes his name, arms, and a wife. Then finally Lleu's misadventures with his wife and Lleu becoming king. Phew!
Two of the Native tales and the three Romances are in the most part older versions of Arthurian legend that differ slightly in the actual contents.
The native tales are:
- The Dream of Macsen Wledig
- Lludd and Llefelys
- The Dream of Rhonabwy
- How Culhwch Won Olwen
- The Tale of Taliesin
These last three are the ones that seem to be related to the Arthurian legends. "The Dream of Rhoabwy" is a late work, in which the main character dreams of King Arthur's time, and may have been written as deliberate fiction. "Culhwch and Olwen", however, is perhaps the oldest Arthurian tale surviving. Taliesin figures in some Arthurian stories as King Arthur's bard, but his tale is thought not to be part of the original Mabinogien.
The Romances are:
- Owain, or the Lady of the Fountain
- Peredur, son of Efrawg
- Geraint and Enid
Apart from the doubtful "Tale of Taliesin", these three were the last parts of the Mabinogien to be composed, probably around the same time as most of the other medieval Arthurian stories, but draw on older roots. Peredur is part of the Grail cycle.
Tropes found in the Mabinogion
- Achilles in His Tent: In Culhwch and Olwen, Cai leaves the band in a sulk after Arthur sings a satirical verse at him.
- All There in the Manual: The Mabinogion is just one part of a much larger collection of epic prose and poetry spanning several centuries of Welsh culture. Events that seem out of nowhere, like the Giant Claw, are explained in other tales, many of which are sadly lost. For some light background reading, check out  and the Trioedd Ynys Prydein ("Triads of the Island of Britain").
- Artificial Human: Blodeuedd. She's "born" when Gwydion and Math club together to find a wife for Lleu, crafting her from the flowers of broom, meadowsweet and oak.
- Badass Crew: Culhwch and his companions in Culhwch and Olwen. They break Mabon ap Modron out of the tower he's been imprisoned in since he was three days old and hunt an enchanted boar with poisonous bristles.
- Bag of Holding: Rhiannon gives a small bag to Pwyll in order to trick her unwanted suitor, Gwawl. It holds an entire feast's worth of food and, when he puts both feet into it, a grown man, with enough space to tie the bag closed over his head. It actually mentions that even if all the food and drink of seven cantrefs note were put into the bag, it would still not be full. One feast, no matter how grand, could not even come close to filling it.
- Came Back Wrong: The Cauldron from the Second Branch that brings people back from the dead. As the afterlife must be a secret they can no longer speak.
- Celibate Hero: Pwyll with Arawn's wife again. Seeing as she did not know about the switch she must have been worried about the marriage.
- Color Motif: White, like the Cŵn Annwn ("hounds of Annwn") in the First Branch, had supernatural associations in early Celtic culture. The name of Arawn's enemy, Hafgan, means "summer white."
- Cool and Unusual Punishment: Gwydion and Gilfaethwy get handed one from their uncle, where they're transformed into a different pair of breeding animals each year for three years.
- Demoted to Extra: Pryderi, in the Second and Fourth Branches. Half of the first branch revolves around his birth, kidnap and reunion with his parents and the third is about his return to Dyfed after the war in the second branch, but he only gets one mention in the second branch and is promptly Killed Off for Real at the beginning of the fourth.
- Engagement Challenge: In How Culhwch Won Olwen, Culhwch is cursed by his Wicked Stepmother that he can marry no-one but the daughter of Ysbaddaden the Giant, who claims that he cannot prepare for the ceremony until Culhwch hunts the giant boar Twrch Trwyth and retreives a comb, scissors and razor from his hair. But he can only be tracked by a certain hound, and the leash can only be made by a certain hero but held by another... until the job involves over forty different tasks and no less than King Arthur and his warband.
- Expy: Children's author Lloyd Alexander trawled The Mabinogion to create the characters, setting, and history of The Chronicles of Prydain.
- Fate Worse than Death: Blodeuwedd is forced to live for all eternity as an owl.
- Fertile Feet: White trefoils spring up in Olwen's footsteps.
- Full-Boar Action: Twrch Trwyth is definitely one of the more badass pigs in world literature; he can't be hunted except with certain hounds, has poisonous bristles and requires the likes of Gwyn ap Nudd (legendary leader of The Wild Hunt), to help bring him down.
- Gender Bender: Gwydion and Gilfaethwy. Biologically male, both of them alternate being female throughout their punishment for the rape of Goewin.
- Handicapped Badass: Bedwyr, the one-armed knight.
- Important Haircut: King Arthur cuts Culhwch's hair in the presence of his court at the beginning of How Culhwch Won Olwen. In Medieval Britain, this was an important gesture of fidelity between family and clan members. Arthur is symbolically accepting his younger cousin into his retinue, which then allows Culhwch to ask for his assistance in gaining Olwen.
- Karmic Transformation: As punishment for the rape of Goewin, King Math transforms the brothers Gilfaethwy and Gwydion in different animals each year, one a male and one a female, until they have borne three offspring together.
- Long List: How Culhwch Won Olwen contains a list of King Arthur's companions which numbers at around 260 names, and takes up about four pages in some editions.
- Magic Knight: Gwydion from the Fourth Branch is one of the most martial wizards in myth. He defeats Pryderi in one on one combat, and he once animated an entire forest of trees to serve as an army against Arawn.
- Meaningful Name: The names of Bendigeidfran's brothers, the loving Nysien and the wrathful Efnysien, mean respectively "friendly one" and "hostile/enemy one". To highlight their differences, "Efnysien" also means "not Nysien". 'Nysien' literally means 'Peace', making their names 'Peace' and 'Not-Peace'.
- Mister Seahorse: Gwydion and Gilfaethwy (two men), are successively turned into a stag and a hind, a boar and a sow, and a pair of wolves. They are in these forms long enough to bear a son from each transformation. After the brothers are turned back into redeemed humans, their animal offspring are then turned into humans and baptized.
- Nigh-Invulnerability: Lleu. He can be killed, but only under extremely specific circumstances. He has to be struck while he has one foot on the back of a billy-goat and one on the edge of a roofed bath that sits on a riverbank. In his own words: "I cannot be killed indoors, nor our of doors; I cannot be killed on horseback, nor on foot." The spear it would take to kill him needs to be forged over a year of Sundays.
- Parental Substitute: Gwydion to Lleu, in the fourth branch; he raises Lleu into adulthood after his mother makes it clear that she's ashamed of his birth.
- Polar Opposite Twins: Branwen's half brothers Nysien and Efnisien. Nysien is a gentle, peaceful man. Efnysien is a sorta heroic sociopath who dies destroying the artifact that was letting the Irish win the war he started in the first place.
- Redemption Equals Death: Efnisien, the catalyst for the entire war, sacrifices himself to clinch victory for the Britons.
- Royals Who Actually Do Something: Pryderi leads his troops into war, as does Math. Pwyll defeats Arawn's enemy Hafgan and cements an alliance between Annwn and Dyfed.
- Rule of Three: Triads, a narrative device in Welsh lore that linked stories together. For example, Branwen is "yn tryded prif riei", one of the Three Main Parents, the other two being Rhiannon and Aranrhod.
- Sociopathic Hero: Efnisien is psychotic, warped, and cruel, but he does ultimately sacrifice himself for the few remaining Britons.
- The Ugly Guy's Hot Daughter: Ysbaddaden, the brutal giant chief, is not much of a looker. His daughter, Olwen, on the other hand...
- Villain Protagonist: In the first act of "Math Son of Mathonwy", the protagonist Gwydion engineers a war with the south (which leads to the deaths of hundreds of warriors, including King Pryderi of Dyfed) only to get Math out of the palace just so Gwydion's brother can rape Math's footholder, Goewin.