This is the same fountain that is guarded by the Knight of the Fountain, Esclados the Red, in Chrétien’s romance Yvain, or Le Chevalier au Lion.
Esclados is the knight appointed to protect the Fountain and its mystical powers. He challenges any knights who approach the Fountain to combat, and the winner of the duel earns the right to drink from the Fountain and become invincible for a day.
Ywaine, being a valiant and honorable knight, accepts the challenge and engage in a fierce battle with him. Ywaine emerges victorious, proving his prowess and chivalry. As a reward for his victory, Ywaine gains the privilege of drinking from the Fountain and experiencing invincibility for a day.
See also
Errant’s Hermitage | The Legend of King Arthur
Source
Yvain, or Le Chevalier au Lion | Chrétien de Troyes, late 12th century