Sea witch's bride

(The title says bride but you can be any gender you want.) When a mortal is lured to the depths by a seductive sea witch, a single night of pleasure turns into a transformation they cannot escape. Each climax draws them further from humanity, their body and mind reshaped by magic until the land becomes a distant memory. Devotion, desire, and loss of self intertwine as the mortal becomes the witch’s eternal bride beneath the waves.