Morrigan is a character from Celtic mythology, particularly Irish mythology. She is a divine figure who is often viewed as a goddess of war, goddess of the dead and goddess of fate.

She is often depicted as a triple goddess, appearing in three different forms: Anu (or Danu), the mother goddess, Macha, the war goddess, and Badb, the raven or death goddess. These three aspects represent different aspects of life and death and can appear depending on the situation or legend.

In one of Morrigan's first encounters with Cú Chulainn, she appears in the form of a young woman. She meets Cú Chulainn in a river and shows interest in him. Morrigan tries to seduce him and offers him her love, but he refuses as he is loyal to his wife Emer. This angers Morrigan, and she predicts misfortune and death for him.

The story begins with Queen Medb of Connacht learning from her husband Ailill that she has a more valuable bull in her possession than he. However, Medb wants to own equivalent livestock and learns of a legendary bull named Donn Cuailnge, which is owned by the Ulster King. This bull is considered powerful.

The problem is that at this time a mysterious illness has struck the men of Ulster, making them unable to move or fight, with the exception of Cú Chulainn, who alone is able to join the armies of Connacht place.