Efnysien fab Euroswydd (also spelled Efnissien or Efnisien) is a sadistic anti-hero in Welsh mythology, appearing prominently in the tale of Branwen ferch Llyr, 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, Manawydan and Branwen. The Welsh Triads call Llyr 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.

Efnysien fab Euroswydd (also spelled Efnissien or Efnisien) is a sadistic anti-hero in Welsh mythology, appearing prominently in the tale of Branwen ferch Llyr, 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, Manawydan and Branwen. The Welsh Triads call Llyr 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.