Efnisien

Efnysien fab Euroswydd (also spelled Efnissien or Efnisien) 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.

The Irish king Matholwch sails to Harlech to speak with Brân the Blessed high king of the Island of the Mighty and to ask for the hand of his sister Branwen in marriage, thus forging an alliance between the two islands. Bendigeidfran agrees to Matholwch's request, but the celebrations are cut short when Efnysien, a half-brother to the children of Llŷr, brutally mutilates Matholwch's horses, angry that his permission was not sought in regards to the marriage. Matholwch is deeply offended until Bran offers him compensation in the form of a magic cauldron that can restore the dead to life. Pleased with the gift, Matholwch and Branwen sail back to Ireland to reign.

...
Wikipedia