Also known as Hu the Mighty, Hu Gadarn was said to have brought the Welsh to Britain from Deffrobani (the Summer Country). There he taught them to plough, and invented the medium of song to aid memory.
After he became king of the first Britons there were said to have been a series of great floods caused by an afanc (water-dwelling monster). Hu's oxen drew the afanc from its domain, enabling it to be defeated and halting the floods.
Hu Gadarn originated in a series of Triads popularised by Iolo Morganwg in the 18th century. Unfortunately the Triads are considered a forgery, and there is little to establish it as an authentic tradition prior to this time.
Although a Huw was mentioned in the Book of Taliesin, there is little to connect the two figures. The name Huw Gadarn does feature in a number of medieval Welsh manuscripts, including the Red Book of Hergest and White Book of Rhydderch, where he was depicted as the emperor of Constantinople, though the tales were adapted from a French romantic tale.
In the 20th century Robert Graves, in The White Goddess, identified Hu with the horned god Cernunnos. Other sources have aligned him with the Celtic god Esus, and on occasion he was known as Hu Hesus, through which Romantics identified him with Jesus.