Half-brother of Bendigeidfran, Branwen, and Manawydan via Penarddun + Eurosswydd; twin of Nisien (foundational good/evil dyad — Nisien wholly peace-loving, Efnisien wholly conflict-provoking). The catastrophic provoker of the Welsh-Irish war: outraged at not having been consulted in Branwen's marriage to Matholwch, maimed Matholwch's horses; during the peace-feast at Matholwch's court, threw his nephew Gwern (Branwen's son) into the fire, triggering the open war. The self-sacrificial cauldron-shattering — hiding himself among the Irish slain dead, being placed in the Cauldron of Rebirth, stretching himself with such force that his heart broke and the cauldron shattered — destroyed the Irish capacity for resurrection but also killed himself. The Efnisien-as-catastrophic-provoker-and-redemptive-self-sacrifice arc is one of the most-complex moral-psychological character developments in medieval Welsh literature; the figure has been read as a foundational case-study of conflict-trauma, sibling-resentment, and the tragic-redemptive cycle.