The Nuckelavee was a powerful Grimm hybrid that wandered around the continent of Anima and made its home in a den near the city of Kuroyuri, before being defeated by Team RNJR. The den was seen in "Kuroyuri".
The den was a large cave filled with Grimm pools and weapons from its many victims. During battle weapons would get stuck in its body and it would come to the cave to shed them off.
The past of the Nuckelavee is unknown, but it has used the cave as far back as the destruction of Kuroyuri as Lie Ren found one of his father's arrows.
Years later Lie Ren and Nora Valkyrie were searching for help and accidentally stumbled into the den. They realized that it is the home of the Nuckelavee that destroyed their early home before hearing it move towards Kuroyuri. They ran to the city towards their friends before being faced with the Grimm.
Image Gallery - Volume 4