The Morrigan

 

There was an extraordinary goddess named the Morrigan, who appears to embody all that is perverse and horrible among supernatural powers. She delighted in setting men at war, and fought among them herself, changing into many frightful shapes and often hovering above fighting armies in the aspect of a crow. She met Cuchulain once and proffered him her love in the guise of a human maid. He refused it, and she persecuted him thenceforward for the most of his life. Warring with him once in the middle of the stream, she turned herself into a water-serpent, and then into a mass of water-weeds, seeking to entangle and drown him. But he conquered and wounded her, and she afterwards became his friend. Before his last battle she passed through Emain Macha at night, and broke the pole of his chariot as a warning.

 

This is taken from T. W. Rolleston's Myths & Legends of the Celtic Race, originally published in 1911.