What is "scathach"

Scáthach , or Sgathaich, is a figure in the Ulster Cycle of Irish mythology. She is a legendary Scottish warrior woman and martial arts teacher who trains the legendary Ulster hero Cú Chulainn in the arts of combat. Texts describe her homeland as Scotland (Alpeach); she is especially associated with the Isle of Skye, where her residence Dún Scáith, or "Dun Sgathaich" (Fortress of Shadows), stands. She is called "the Shadow" and "Warrior Maid" and is the rival and sister of Aífe, both daughters of Árd-Greimne of Lethra.