Efnisien fab Euroswydd is a sadistic anti-hero in Welsh mythology, appearing prominently in the tale of Branwen ferch Llŷr, the second branch of the Mabinogi. Described by Will Parker as "a study in the psychopathic personality" and an "embodiment of the forces of anti-social disruption," he is the catalyst of the tale's ultimate tragedy, and is largely responsible for the destruction of both Ireland and the Island of the Mighty. He is the son of Euroswydd and Penarddun, twin brother to Nisien, and half-brother to Brân the Blessed, Manawydan, and Branwen. The Welsh Triads call Llŷr one of the Three Exalted Prisoners of Britain for his captivity at Euroswydd's hands; this is likely to a lost tradition of the birth of Penarddun's younger sons.

Branwen ferch Llŷr; "Branwen, daughter of Llŷr" is a legendary tale from medieval Welsh literature and the second of the four branches of the Mabinogi. It concerns the children of Llŷr; Bendigeidfran, high king of Britain, and his siblings Manawydan and Branwen, and deals with the latter's marriage to Matholwch, king of Ireland. Matholwch's mistreatment of the British princess leads to a mutually destructive war between the two islands, the deaths of most of the principal characters, and the ascension of Caswallon fab Beli to the British throne. Along with the other branches, the tale can be found in the medieval Red Book of Hergest and White Book of Rhydderch. It is followed directly by the third branch, Manawydan fab Llŷr.