Yvain, or the Knight with the Lion
Yvain, the Knight of the Lion is a romance by Chrétien de Troyes. It was probably written in the 1170s simultaneously with Lancelot, the Knight of the Cart, and includes several references to the action in that poem. In the poem, Yvain seeks to avenge his cousin Calogrenant who had been defeated by an otherworldly knight beside a magical storm-making fountain in the forest of Broceliande. (Summary from Wikipedia)