Lady Evica is one of the prides of the Hesayan Church--it shows that even monstrous, overtly sexual, colossal mermaids can be brought to worship in Iasu's light. Except not really.
The undisputed head of the Trinity, and the story of Falhath’s oldest folk hero.
The God has two faces, and wears two masks.
One is a bright mask, shining with light, shining like the sun’s gold; it is the Gold Mask.
The other is a dark mask, blackened with shadows, black like the night. That is the Dark Mask.
-First Explanation, Priest’s Teachings 1:2, The Book of Two Faces
If you want to add a "supernatural" touch to your campaign, define the rules of magic and the universe. Make your players comfortable with those rules. Then your new supernatural creatures must then break those rules.