Above pictured is one of my very favorite portrayals of the Undine. (Or, in germanic folklore, Ondine.) It is said that the Ondine are ever in search of what they are missing -- a soul. They are water elemental spirits, or nymphs as they are often called. It is also said that the only way that the Ondine can gain a soul is to marry and bear a child to a human. But, in exchange for her soul, she loses her immortality and has to take on the suffering of an ordinary human. In the event that, once she begins to age, her human husband is unfaithful, she may curse him. He vowed to love her with each waking breath -- So, he would fail to breathe if he fell asleep ever again, and so the curse would kill him. They tend to be tragic figures, I think -- which brings me back my novel:
One of the characters in my novel is a Siren -- so to speak. More like a combination of mermaids and sirens, but not in the way that disney's Pirates of the Caribbean: On Stranger Tides portrays mermaids. The Sirens of my book do take human form on land, but in the water, they take mermaid form. On land, they do not have the powers they would have in the water; they cannot sing to enrapture a sailor/pirate. In the water, their songs enchant the male sailors, pirates, ect to abandon ship, and so drown, ever enchanted by the song, never realizing their own doom.
Then, there is a combination of the above, with the idea of the Lamia, which is a serpent-like demon in several mythologies. In some, she is a child of Zeus and twisted by Hera's hatred/grief to become a demon that ate children. They enchant and seduce in some, where in others they keep items or information from the Hero -- who would otherwise fall victim to her powers. In still others, she is but a half-human, half-serpent monster who lives in filth and stupidity.
My character, because she has some of the same traits as my Sirens, has an enchanting singing voice, but she only sings of grief and suffering. Those who hear her song and are not strong enough to overcome mostly die due to their inability to comprehend and fend away such grief. There will be more on that in the story. Sybaris, as I have named her from the mythologies, is of water. She does not travel much from her cursed isle, but she can if she needs be. She is an ugly serpent monster to all but a few, who see her as a human -- except for her eyes, which are entirely black, and her movements, which are not humanly graceful, but almost...twitchy...kind of like a snake. Most, if not all, fear her, but she doesn't kill on purpose. It is her nature, and powers she was born with that cannot be controlled. At her birth, which should not have been, she took control away from the Gods over the entire ocean. Chaos ensues in several ways, and though she tries to die to give back the natural balance of things, she cannot. She is immortal and cursed. It is the Hero's job to figure out how to restore the balance of things in the wake of the man who caused this event to occur in the first place -- the pirate captain who captured him. I won't say more. I'm still writing anyway. All of the above are tragic creatures, which helped me to figure out the most strange "antagonist" (who isn't really entirely an antagonist) of my book.
But I wanted to just touch on the fact that though mythologies are pretty solid foundations, they can make interesting new monsters or characters with just a bit of planning and creativity. There's lots that can be gained from myth -- not just religious things, really -- but also plenty of material that can be worked into a creative work somehow.