Efnisien

Efnisien appears within the Second Branch of the Mabinogi (Branwen ferch Llŷr) as a member of the royal household of Llŷr: a son of Euroswydd and Penarddun, and half-brother to Brân the Blessed, Manawydan, and Branwen. His role in the tale is not as an otherworldly origin-figure but as a human agent whose personal rage and actions—beginning with an insult over Branwen's marriage—drive the narrative toward catastrophic conflict between Britain and Ireland.

Medieval sources and the supplied summaries do not provide a sustained physical description of Efnisien (height, dress, or distinguishing features). Accounts emphasize his behaviour—violent deeds and fierce initiative—rather than bodily detail. Genealogical position is emphasized: he is twin to Nisien and half-brother to Brân and Branwen, situating him within the royal family.

No explicit supernatural powers are attributed to Efnisien in the supplied materials. His notable capacities are extraordinary violence, physical strength (e.g., crushing the skulls of hidden warriors), cunning suspicion (reconnoitring the hall to uncover treachery), and single-handed destructive initiative (mutilating horses, burning Gwern). He also performs a deliberate self-sacrificial act by hiding among corpses and being cast into the Irish cauldron, destroying that magical revivification device from within at the cost of his life. The cauldron itself is the only clearly magical object noted in the tale.