Yeats was beginning to use a vocabulary freshly minted from the treasury of Gaelic literature, and many of the shorter poems in The Countess Kathleen and Various Legends and Lyrics (1892) deal with a mythology Ireland had well nigh forgotten and England never known. For Arthur and his Round Table Yeats substituted the very different Conchubar and his Red Branch Warriors, and Finn and his Fenians. The Red Branch cycle of legends included Fergus, whom Ness had tricked out of his kingdom so that her son Conchubar could rule over Ulster in his stead, and in Fergus and the Druid Yeats makes him avid for dreaming wisdom. Fergus was the unwitting agent of the doom of the Sons of Usna, Naoise the lover of Deirdre and his brothers Ardan and Ainle, who had accompanied the lovers to Scotland when they fled from Conchubar’s wrath, for Deirdre was Conchubar’s intended bride. Fergus had persuaded them to return against the wishes of Deirdre and had been tricked out of acting as their safe conduct. He joined with Maeve, Queen of Connaught, after this, in her raid on Ulster, in which Cuchulain achieved his great fame as Ulster’s champion. Cuchulain is the Achilles of the Irish Saga, and he appears throughout Yeats’s plays and poems, as warrior, as husband of Emer, as lover of Eithne Inguba, and of Aoife, as the unknowing killer of his own son and finally as victim of the sea.

Cuchulain is called the "Achilles of the Irish Saga" because ______.