Cú Chulainn's figure was mutated by means of Queen Medb's desire; he is a Servant reflecting the wish made upon the Holy Grail by Medb "for a wicked monarch similar to both myself and Cú Chulainn." He wolfs down on the land of America as the Mad King Cú Chulainn (狂王クー・フーリン, Kyōnō Kū Fūrin). A calamity-class enemy from the Fifth Singularity of Fate/Grand Order.

(Source: TYPE-MOON Wiki)