Dwyfan and Dwyfach, sometimes also called Dwyvan and Dwyvach, in Welsh mythology, were the equivalents of Noah or Deucalion who take their names from small rivers, as told in a flood legend from the Welsh Triads. A great flood was caused by the monster Afanc, who dwelt in Llyn Llion (possibly Bala Lake). All humans were drowned except Dwyfan and Dwyfach, who escaped in a mastless boat. They built an imposing ship (or ark) called Nefyd Naf Neifion, on which they carried two of every living kind. From Dwyfan and Dwyfach all of the island of Prydain (Britain) was repeopled. Dwyfach appears to take her name from the small Dwyfach (Welsh: little Dwy) River of Gwynedd (until 1974, Caernarvonshire) that flows into Cardigan Bay; Dwyfan would then derive from the river it enters, the Dwyfawr or Dwyfor (Welsh: great Dwy).A lake monster from Welsh mythology, the afanc can also be traced through references in British and Celtic folklore.
Sometimes described as taking the form of a crocodile, giant beaver or dwarf, it is also said to be a demonic creature. The afanc was said to attack and devour anyone who entered its waters.
Various versions of the tale are known to have existed. Iolo Morgannwg, who revived Welsh bardic traditions during the 18th and 19th centuries, popularised a version of the myth that had Hu Gadarn's two long-horned oxen drag the afanc from the lake, enabling it to be killed. An earlier variation on this had the oxen cast the afanc into Llyn Ffynnon Las (lake of the blue fountain), where it was unable to breach its rocky banks to escape.
In one telling the wild thrashings of the afanc caused flooding which drowned all the people of Britain, save two, Dwyfan and Dwyfach. Another has a maiden who tamed the afanc by letting it sleep in her lap, which allowed her fellow villagers to capture it. When the afanc awoke its struggles crushed the maiden.
Later legends had King Arthur or Peredur slaying the monster. Near Llyn Barfog is a rock with a hoof print carved into it, along with the words Carn March Arthur (stone of Arthur's horse), supposedly made when his steed, Llamrai, dragged the afanc from the deep.
The afanc has been variously known as the addanc, adanc, addane, avanc, abhac and abac. Several sites lay claim to its domain, among them Llyn Llion, Llyn Barfog ad Llyn-yr-Afanc (the Afanc Pool), a lake in Betws-y-Coed.