Scáthach was a formidable tall Scottish warrior woman with fiery red hair, who taught mighty warriors in the arts of combat. She taught from her fortified residence Dún Scáith on the Island of Shadows in Alba (Scotland). Today the island is still called the Island of Scáthach or Skye. She herself was known as “the Shadow” in reference to her ability as a stealth warrior. She is sister and rival to Aoife, and daughter of Árd-Greimne of Lethra.
Chieftain Forgall Monach set Cú Chulainn a number of tasks to prove himself worthy to marry his daughter Emer, his beloved. Cú Chulainn is sent to Alba with Lóegaire, Conchobor and Conall Cernach, to receive training from the warrior Domnall. Domnall had a hideous daughter who sought revenge on Cú Chulainn when he refused her advances. Eventually, Domnall sends Cú Chulainn to Scáthach to complete his training, recognising his superior abilities.
Cú Chulainn then travelled to Dún Scáith, taking along his companion Ferdiad. Scáthach taught them in the use of arms, and even gave Cú Chulainn her deadly spear, the Gáe Bulg (“the spear of mortal death”), made from the bone of a sea monster, the Coinchenn, that died whilst fighting another sea monster, the Curruid. The Gáe Bulg was a deadly weapon that entered a man's body with a single wound, like a javelin, then opened into thirty barbs. When roused, he entered into a terrifying battle frenzy, the ríastrad (warp spasm), which was both a blessing and a curse. His appearance changed and he became an unrecognisable monster, who could not distinguish between allies or enemies.
Whilst at Dún Scáith, Cú Chulainn, had an affair with Scáthach's daughter Uathach, but accidentally broke one of her fingers. Uathach's suitor, Cochar Croibhe, challenged him to single combat despite Uathach's protests. Cú Chulainn killed him with ease and became Uathach's lover. To make amends to Uathach and Scáthach, Cú Chulainn also took on Cochar's duties. Scáthach promised her daughter to him in marriage.
One day, Scáthach's rival, also a warrior woman, her sister Aoife advances on Dún Scáith. Cú Chulainn fought her sister and defeated Aoife at swordpoint. He spared her life on the condition that she will lie with him and bear him a son. Aoife later gives birth to Connla. Having become a mighty warrior. Cú Chulainn finished his training with Scáthach and returned to Ireland, abandoning Uathach. Later when Connla matured, Cú Chulainn requested his presence in Ireland.