The Eldritch Ettin is located on the third level of the Bloodstone Caves and is the final boss of that dungeon. Along with his own mob of Enchanted Shields and Enchanted Scythes, there are two additional mobs close by consisting of a First Inscribed Ettin, and more enchanted weapons. It is not necessary to defeat these other groups to complete the quest. Defeating him will drop the spectral crystal and reveal the Eldritch Chest.