Scion created to rid the world of its impurities by swallowing them within himself, in opposition to Nabriales the Majestic, scion of light. The world, however, was more filled with impurity and corruption than even the gods dared imagine, and having swalled it all, the once beautiful Cuchulainn was transformed into a hideous thing, a deity of filth, and so did he turn against his creators. Wherever his feet should fall, there all life withers to dust.
To find this Esper, the party must complete the "Lost in the Pudding" hunt to get the Sluice Gate Key. Next, complete the "Waterway Haunting" side quest to adjust the water levels and open a path to the boss.
Cuchulainn is an easy Esper to use. He has plenty of HP at his disposal and often heals with Curaja, which enables his summoner to focus more on combat and less on keeping the Esper alive. When paired with a powerful melee character, they can devastate any lesser enemies that crawl around nearly any location.