Which Irish hero is known for his warp spasm?

Cú Chulainn is the Irish hero known for his warp spasm. Called ríastrad in Irish tradition, this battle frenzy transformed him into a terrifying, superhuman figure in stories of the Ulster Cycle.

The best-known description appears in the medieval epic Táin Bó Cúailnge, or The Cattle Raid of Cooley. When the invasion of Ulster begins, Cú Chulainn is the chief defender able to fight because a supernatural curse leaves the province’s other warriors incapacitated. His ríastrad amplifies his strength and ferocity beyond ordinary human limits.

Descriptions of the transformation are deliberately grotesque: his body twists, muscles swell, and his appearance becomes monstrous. “Warp spasm” is a famous English rendering, especially associated with Thomas Kinsella’s 1969 translation, while ríastrad is the traditional Irish term.

Fergus mac Róich and Conchobar mac Nessa are important Ulster figures, but neither is defined by this transformation. Fionn mac Cumhaill belongs mainly to the Fenian Cycle, a separate body of Irish heroic legend.