Morsa is the evil goddess of death. She is worshipped by Lucia and Kharis. She was also Neria's enemy.
Serafina and Ling find a temple to Morsa in the ruins of Atlantis. When Serafina pulls a bloodsong from a pool of blood, she hears disconcerting screams. Orfeo probably worships Morsa so he can get his wife Alma back from the dead.
Lucia takes Bianca to a shipwreck with a temple dedicated to Morsa. This is where Kharis gives Lucia a maligno that looks like Mahdi.